In preparation for my ultra next June, I fitted some cheap tri bars to my adventure bike. Initially, they felt sketchy, and I wasn’t sure if I’d want to keep them, but people running them do so for a reason even if that reason is just somewhere different to stick your arms.
I’ve been on the cycle path doing lots of z2 and practising using them (comfort might take a while it’s some new muscles to work) but on a 5.7km lap of a reservoir, I set a PB and had a ~0.5km faster average speed but only 83W instead of 96W. I reckon even if I’m only on them for 30km or 40km out of the 320km it’s going to really be beneficial.
A lot of Audax / Randonneurs / Ultras use the aero bars primarily to give their hands and wrists a break. Spending 12-20 hours per day with weight pressing on your ulnar nerve is a recipe for months of numb fingers.
Personally, I have my aero bars jacked up so that it’s the exact same hip angle as when I’m in the drops. It’s a limited amount of aero gains (just the advantage of having the arms tucked in) and I get funny looks from any triathlete, but it’s very comfortable for long spells of flat road riding.
Indeed, aero gains is half of story, other side is supporting body over long durations. But it does not come without price, during initial weeks you may notice neck soreness and discomfort looking upwards when head is lowered. In couple weeks it should get better, assuming there are no other bike fit issues. Same story is when you get rid of those bars: initially triceps and lower back may be sore, and it takes little time to get accustomed riding on hoods or drops again.
For that reason I use tri-bars for long continuous period (April-July) when doing long rides. Rest of year riding without them while training with different focus (pushing TTE/FTP, etc).