Turboweasle, I'm not sure how much you play the conduit but...
Streets: stay away from the windows , throw nades at stairs, go back to ammo spot, repeat (streets isn't as bad as other maps tho).
So don't go up the stairs. Throw a flash through the second-story window and come in through there. Problem solved. Also, maybe you should spam nades through the windows yourself. Just a thought.
Complex: how can you get them from a range? the grenades are like perfectly set up to get spammed at, there's a high point (which you can't shoot the other high point from because your too high up to see them), a low point, where you get nade spammed to death, and then the other high point where the nade station is.
So don't go down into their base. Most of the fighting is in the middle of the map, where the SCAR is your main problem. Problem solved.
Pentagon: Depending on how smart the team is, you can grab the grenades and then camp back in the rooms near where the ASE would be in team objective.. going to restock ammo you could get sniped yes, but just because people are nade spamming doesnt mean they don't have a gun either, they can snipe you back too :|
So be the better sniper? Spam nades into their base? If they're dumb enough to allow themselves to be bottled in, use that to your advantage. There are two ways to get into their base, as well as two floors with very good vantage points for sniping that go directly to their base. Use them. Problem solved.
Bunker: Yeah, no way you're going snipe someone here.... enough said.
Are you kidding? Bunker is a great place to snipe. It's got two floors and windows everywhere. It's a sniper's dream.
Sanctum: Yeah, the one upstairs isn't a huge issue... easily snipe them there... but then high voltage thought to themselves "how the hell are they gonna nade spam on this map", so they decided to put one right in the bottom floor so people could camp there and spam.
Map control. Get there first. Or you could spam nades back.

Infirmary, sniping here?? Not going to happen... grab the grenades chuck them at hallways and run back to your safely wall-surrounded Ammo box.
You're right-- long, straight hallways are terrible for sniping.

Fact is, we allow grenades... it will turn into a match like this:
My team kills yours first, we run back to the ammo, we camp... and we're at a restock advantage.
So don't let that happen. Get the first kill or learn how to counter. You can nade-spam too, you know.
If you can't win with grenade you fail **** plain and simple...