My thoughts on this are:
If the assault can beam down on target, with an essentially unlimited number of troops, given enough time, no need for snipers, just beam down the army.
If the target is tougher, it will be softened up by orbital bombardment.
If the target can sense life forms at ranges of kilometers, snipers cannot close to effective range.
One use i can see for a sniper would be a hostage situation / Standoff, in a city, but still, beaming in by a few dozen security guys in security armor with weapons would more or less nullify that, as would wide angle stun, from a ship, say in a few blocks radius.
Only if you had some deal where there were hostages inside a shielded area, with hostage takers heavily armed, for some type of political goal, since money doesn't exist, right? Hey, what a scenario.
You might have a case for one or more snipers with that setup. However, given okay, by Era, Betazoid mind reading, etc. ... well, it sounds like an interesting scenario.
But it seems like unless you got some kind of Tal Shiar / Obsidian Order op going on inside Fed space, most bad guys are gonna be off on the frontier, pirating mining ships, or something, and definitely dodging anything like a Starship's crew, or anywhere near where one could be deployed.