I think we started with random instead of fixed on this list, but I'm not sure where they stand now. If we are still on random, then it is certainly a point that relying on a stat being a certain level, or worse yet 2 or 3 stats, and it's each like a 50% chance of getting there, giving a couple of extra levels definitely has merit. It's like, really unlikely to have the requisite str and spd to kill something at the level averages tell you they'll have it. Sure, you could randomly get 4 more spd than you need, but that doesn't help much when you have a chance of falling one str short. If you have a 50/50 chance to get 12 str and 50% for 17 spd and you need both, you pretty much have a 25% chance of actually being able to kill the unit if you need both 12 str and 17 spd.
On the other hand, there are stat boosters at various points and you can use a forge more often on one character that got stat-screwed. Still, though, having a few extra levels is good.
And though I'm not sure why you quoted me there, it kinda works. That bexp spent on Mia for chapter 9 alone is never helping you afterwards. Unless you use her, that is, but lots of people say mount rush the game so not sure what Mia does with 7 move beyond that point. Now, the point dondon brought up is "does it actually cost you anything?" It most certainly costs in reliability, I'll give you that, but sadly lots of people here seem to value 4 turns at 70% much much higher than 5 turns at 99%, so I kinda stopped caring long ago. My soul was saddened by my fave characters being better at reliability than super-low turns and super-low turns taking precedent.