100%! Yes, indoor trainers can take the place that road bikes used to have in XC MTB training.
True. Although many people would 100 % prefer riding outdoors if given the chance. (Not me, personally, but I know and know of plenty of them.)
I would mostly agree but with a small caveat. I use a trainer over the winter and come the Spring venture outside on my MTB.
Performing Vo2max or threashold intervals up a bumpy trail is very different to on a trainer, I find it takes me a month or so to match my interval trainer power when on the trail.
I guess itâs a mix of the extra concentration needed plus the extra movement of the bike / body.
I used to train exclusively on my mountain bike, but eventually in year two or three, the wear and tear on the bike was quite high and then it makes your training intermittent if you need to do a major service like front fork or shock and replacing cassettes and chains. Although some of those are relatively easy, supply chain issues the last few years has added to the time your bike might be out of service and thus training is interrupted even further.
I know everyone jokes about N+1 but this is a real reason whyâŚobviously, not everyone can afford or have space for a fleet of bikes (or even a second one) but having some sort of back up option is really handy. Itâs also a major advantage to doing your own service, your bike is our of commission less time
Iâm down to 2 bikes. A gravel bike and a mountain bike. The gravel bike serves as a road bike if needed.
I do most basic service on it except for major stuff. But my LBS always has minimum week turnaround even for somewhat minor stuff so having that extra bike is nice. Granted I have a Kickr bike so Iâm never truly out of being able to train but could be out of outside riding if I only had 1 bike.
In addition, mtb trails will close when itâs wet so the gravel bike still allows me to get out and ride.
And motivation wise itâs nice having 2 bikes with different purposes.
This is such a good point, and I am thankful every time that some mechanical issue or me waiting on a part does not simply stop my training. And IMHO when you get a second bike, it should be a bit different from your other one. A road bike is definitely different from a mountain bike 
I once read
Your mountain bike is your best friend. But your road bike will always be your lover.
Agreed - I definitely train intervals both inside and outside. It does take a lot more concentration and route planning to pick the right places and often thereâs a lot of extra riding to get to the right hill. So, in foul weather or when time-cruched itâs great to have the trainer as an option. But itâs also great training to ride mtb on trails especially during the specialty phase in the lead up to a mtb event.
I think same thing applies when using a road bike. You want to prioritize the mtb again in the leadup to the MTB event.
immediately no.
In all seriousness the answer is obviously no. A mountain bike will happily ride on the road, if I put semi slicks on mine I can average 17-18mph on the road (sweetspot-ish), obviously not as fast as a dedicated road bike, but still riding on the road.
That sounds terrible
I think on the road a road bike will provide a safer and overall better training platform
It could be argued that a wider tyred bike that will be more stable and slower for a given target power is safer. Iâve known a few good roadies continue to train on a mtb when it has become to grotty and unsafe for a road bike. Iâm not sure about the position though and myself Iâd train indoors in those conditions. Then again perhaps thatâs why I am not a winner like them 
Safer?? On the road I donât have to worry about potholes, poor surfaces, gravel and with 2.2 tyres at 24psi itâll be more comfy.
Granted it wonât be as fast or as nimble as a road bike but it will do the job no problem.
No, you donât need a road bike just to ride on paved surfaces. I donât know what your friend means by âmaximise your trainingâ⌠If XC racing is your focus, youâll maximise your training by training on your XC bike.
For me, I have a nice long flat bike path I can do my structured rides on. I usually keep a big 38 tooth ring on the XC bike for these rides, then when race day comes I throw on whatever ring is needed for that race.
I guess I just donât feel like a MTB is comparable to a road bike on the road. The road bike has way narrower bars and is designed for riding on paved surfaces at high speeds. Iâm a former bike messenger though so Iâm coming at it from a different perspective but IMO a MTB on the road is totally possible but itâs just not ideal at all for so many reasons. If the point is to train to get stronger Iâm positive it will be easier and more effective on a road bike.
The other thing is if you want to ride or train with other people to improve guess what you arenât gonna be able to use your MTB on a group ride. So if you want to ride slowly on the road by yourself a MTB is fine I suppose.
Most people train alone. Riding in a group is rarely proper training and couldnât be further away from MTB.
Maybe a slightly different take on it from my personal situation. We have decent mountain biking around here but for me, it is a 45 minutes drive (at best) across town to get to the trailhead, more often it was closer to 60-70 minute drive. Loading the bike, the gear, the drive both ways, trying to keep the car clean made more than once or twice a week a logistical hassle with work, kids, home life. Having a road bike, I step out my door and go any time I have free time, short rides, long rides, training, or just tooling around the neighborhood.
The last thing I want to do is persuade people they donât need another bike 
I guess most of the items are personal preferance / riding history, however the training effect is the same, 250w for 2hrs is the same training effect regardless of MTB or roadbike (youâll just have travelled further on a roadbike)
Right or wrong, I think he was saying that the road bike can be a more âeffectiveâ tool in regards to training. Longer endurance rides are more common on the road, intervals and structured training can be done easier on the road, etc.
As of now my fleet is all MTB⌠though I did get a Kickr bike to use for TrainerRoad. I can see the trails from my house so itâs what I ride 95% of the time. Iâm sure a road bike is in the future at some point though⌠I have this thing where I like buying new bikes. 