Well, I was just thinking that, as the game goes on, the initial units or those stationed in cities have no way to receive a little bit of XP.
The same for the monsters...as the time goes on, they could benefit of a little of upgrade.
So if any unit gains a little XP just for being alive in a devastated world, could add a bit of flavour...and feel the world more alive. It is important for units with low level, but will not affect to high level units, as they need lots of XP... It is also aimed for the early units, the first ones in get obsolete...they will get a bit more of benefit...
+1 xp/turn means upgrade from level 1 to level 2 in 15 turns (almost 4 years), but after that, the number of turns increases in a way that, after long games, the level for stationed troops is very few...
I think the only affected building is the +25% chances of +1 XP for stationed heroes each turn (Adventurers Guild?). That bonus is rather crappy, so it has little use anyway. And only affects to heroes... It could be buffed...or not, it doesn't matter.
So what do you think about it? Will it affect too much to the game mechanics? Is it not well justified?