Gut bacteria have beneficial effects, such as the ability to break down foods which would otherwise remain indigestible. If birds and bats have reduced gut microbiomes then it suggests they have a reduced ability to digest certain foodstuffs.
Also: the weight loss referred to is the reduction in gut bacteria mass.
Sure, it's a positive effect on your ability to fly. But if you do anything other than flying, it will affect those things too. It's terrible for your ability to recover from injuries.
