Trainer is not just about wind, it's also about trigger time. I actually might argue that it's more about trigger time than wind skill, especially for positional/PRS style barricade shooting.
Cost wise the difference between shooting a 6.5 and a 223 is roughly $0.40-50 per round, once you factor all costs including barrel wear. Shooting 4,000 rounds per year that's about $2k cheaper per year to shoot 223. So even if you go full bore and build a complete $6k 223 match rifle as a trainer the break even is in three years. But the reality is that most of that $6k would have depreciated only maybe 30% after five years. You could still part the gun out and sell trigger/chassis/action/scope/rings separately and get most of your $6k back. Reality is you probably break even on non-depreciated costs within the first six months. For a switch barrel rig it's almost an immediate payoff, since barrel wear is a constant incremental cost. Sure you might have more cost tied up in barrels (ie more inventory) but the cost benefits start immediately since you're putting the "cheaper" barrel wear on your trainer from day 1. Stupid accounting tricks aside, to me there are other bigger benefits of a trainer...
One is that you don't have to mess with your match gun once it's dialed in. No worries about barrel speed changing, lands eroding and needing to tweak seating depth, whether the barrel needs to be cleaned but then fouled before a match, does it have enough life in it to shoot the next three matches or do I need to stop practicing, is it a brand new barrel I just spun on so I need to break the barrel in and do load development but I've only got 2 weeks until the next match. All that crap becomes "work" eventually, so once you have a match rifle dialed in... don't touch it and leave it ready for the next match.
Second is that you can feel way more free to abuse a barrel on a trainer than on a match rifle. Shoot faster, let it cool less, worry less about cost as you're pulling the trigger, worry less about being meticulous about brass preparation or powder measuring or counting the number of reloads on each case or whatever. It's cheap ammo that you can load quickly on a progressive and throw into a bucket and go to the range. Less stress, less hassle, less recoil, simpler range time.
I also love that 223 is way slower to heat up the barrel than a 6mm or 6.5mm. A typical range trip for me might be 80-100 rounds in under an hour working a barricade. Load a mag about half full, shoot some positions, walk back and reload another mag, keep shooting. At that pace the barrel never gets too hot, at least on cooler days we're having right now. You couldn't do that with a match gun in a larger caliber without it getting seriously hot.
Finally the nice thing about having a complete trainer rifle is that it's ready to go. Got a spare hour or two? Grab the gun and go shoot, no barrel switching, no rezeroing, etc. Makes things logistically easier, so you're more likely to shoot and/or you get more time shooting when you go.
Circling back to the original question of 223 vs 223AI. For me I went 223 because it was simpler/cheaper because I already had a nice set of 223 dies on hand. Not interesting in hot rodding, my trainer load is 23.2g of XBR8208 behind a 75gr BTHP at 2810fps from a 26" barrel - super conservative. Was still drilling a 10" plate at 800 yards at will yesterday during barricade practice. I kinda wish for a 223AI to avoid trimming brass, but I guess I'll just have to get a Giraud trimmer.