Well, for many things, there is a finite amount of experience to be gained in a given mission. For a given set of weapons, there will only be a certain number of hits until the aliens are dead, which means there is a cap to how much firing accuracy XP you can get.
IF (and I don't think it works that way) you can get secondary XP from targeting an alien and missing, then that's an unlimited XP supply. But I would be very surprised. I am fairly certain that when shooting an alien in your turn, you need to hit it to get XP. Even though the chance to hit goes down, the total number of hits to kill the aliens remains similar, it will just take a little bit longer. So you get the same amount of XP.
Regarding reaction fire, the accuracy extender is actually a bonus XP wise. Reaction fire gives XP no matter if a shot hits or not. If a shot hits, the alien is closer to death and a dead alien is worth no XP. Accuracy extender makes snapshots less accurate past a certain distance, which will not reduce reaction XP (firing is what matters for reaction) but increase the survival chance of the alien (you're more likely to miss it). That means another soldier might react to the alien and also gain XP. Of course, however good it is for XP, missing aliens is bad for your health
The only case where accuracy extender is bad for XP is IF missing on your turn when shooting gives XP and you argue that you will have to use aimed shots so you will overall miss less than using snap shots without. You could either not game the system, rise to the challenge and use snap shots within 15 tiles (where snap accuracy is the same), or keep using snap shots and miss a bit more (which would mean more XP IF you get XP from missing, which I doubt).
Overall, extender accuracy does make the game harder, but it also makes it much more interesting and makes it possible to differentiate weapons greatly. Snap shooting across the map as reliably with a pistol, machine gun and sniper rifle is ridiculous to me, accuracy extender allows me to represent that in game.