Yes, totally useful. Knowing what kind of efforts are necessary for whatever riding/racing you want to do is always going to be useful.
The thing that is a little trickier is TSS. The power meter isn’t going to adequately capture the stress on your body from, for example, hauling a heavy enduro bike down a tricky descent.
I think MTSS is probably a holy grail for some people and I’m sure there are some smart people working on it. My guess is that to really crack it we’ll need to bring in data from another source (possibly something like the shockwiz), or at least get a three-axis accelerometer on a powermeter (assuming that most PMs have only a two-axis to measure cadence) in order to measure g-forces. How that correlates to stress on your body is a whole other question, though.
When I go for a hard MTB ride with a lot of aggressive descending I usually assume my ‘actual’ TSS is ~5% higher than what gets calculated from my power data.
I use a 4iiii on an XT crank arm on my Santa Cruz Bronson. I am in the market for one of those fun ‘downcountry’ bikes. I’ll either put another 4iiii or a quarq on that. I have a quarq on my CX bike.