At 15 months, you're past the real stage of worry (like suffocating because your baby who couldn't lift their head up rolled into the bumper). That's the main reason people take them out or don't use them at all. Bumpers were invented to keep kids from sticking their appendages into the bars. We now live in a world that the distance between bars in a crib is measured and deemed "safe" by regulations, so essentially bumpers are now habit and decoration.
The other "worry" is that your child might be able to climb out with a boost from the bumpers. We had bumpers in our crib for my daughter (6 months old to 18 months old) and she never ever crawled up on them once to try to get out. She was used to them there, and never stood on them. Besides, the ones we had kinda squished down when you put any kind of pressure on them, so it wouldn't have been a very productive way for her to get a "boost" anyway!!
I think at this stage in his life, it's really your choice to keep them in or take them out. He's old enough to be past the worries of suffocation. If you think he'll use them to climb out, take them out. Other than that, they're a nice decoration.