I think it'd be more fun to not be able to tell where the sniper is right away and be forced to be more cautious/sneaky and also rely more on callouts. Really emphasizes the need of power weapon control even more. In something big like BTB/WZ it's more of a casual experience so I don't mind getting that information for free and you're also on much larger maps so it's nice to have that help locating the sniper.
If you're in a position where you would see the glare, this means that you will be in combat with the sniper within a second ("combat" including you being shot by the sniper even if you didn't see them), because the glare means that the sniper is already taking aim at you. For what you're thinking of, whether there is glare on the sniper scope is totally irrelevant on a small map, because on small maps there is no hiding: if you can see the glare, you can see the player that is emitting the glare. There is no advantage in being cautious when there is no scope glare relative to when there is. Consequently, it doesn't really affect the importance of power weapon control in any meaningful way.
If you want to find someone use your eyes. If players are consistently having trouble picking out players from background or picking out certain weapons that is a failure on the developers part that needs a legitimate rework not tacked on bandaid solution.
This same argument applies equally well to bullet trails. Should we get rid of them, too?
There are two advantages that you're brushing aside here. First is faster recognition. If you deem this a "bandaid solution" to some perceived issue, you're missing this one. It's not that there needs to be some problem where players have significant trouble seeing other players from the background, but that this makes the recognition of the player (and in particular of their weapon) that is aiming at you faster, which is something you're not really going to be able to accomplish by any other means. Or at least the other means are substantially more obnoxious. Of course, another question is whether faster recognition is something you want, but since Halo has from its inception used very bright high contrast colors for its player models, it would seem that this is an integral part of Halo. In fact, this is a good feature for all the same reasons that less visual noise in map design is a good feature.
The second advantage is that unlike most other means of making players more visible, this works at a distance, which is very important for big team gametypes with large maps. Again, this is quite a subtle way of letting the player notice a sniper who is hiding behind some rock with only their head sticking out, who would've certainly gone unnoticed in all previous Halo games unless the player also had a Sniper Rifle. This, in turn, as I and UEG ShadowAngel pointed out, encourages snipers to be more mobile, which at least to me is an integral part of sniping in Halo and how it differs from other shooters.
The arguments against this feature to me seem mostly aesthetic (which is fine, I guess, but not very convincing), and the kind of "back in my day we had to walk uphill both ways" which assumes that because something was more difficult in the past, it shouldn't ever be made any easier.