1) I think everyone besides Sisters (and remember that Sisters were initially meant to be an ally codex, like old daemonhunters) has at least two (base, no FOC swap) troop choices, and at present, I think those two are usually pretty balanced for most of the armies. People have their favorites -- Kroot are often considered a steal over Firewarriors, but no one would argue FW don't have their advantages, for instance. CSM troops are pretty bad -- Cultists are cheap but useless, the Chaos marines themselves are two expensive compared to normal marines with ATSKNF.
2) You can't spam troop choices, by definition, if you ask me.
Arguably, the great strength of marines is the generic usefulness of tac squads. You can have an OK army that was nothing but troops, that's not really possible of any other codex.
3) 100 pts. 12-25 pt units are really rare, but a lot of troop choices can be bought for around 50 pts, and the big fancy uses can get up near 300 pts. (450 for terminator troops)
4) Mur? All armys need troops. All army's I think you could make two different functional lists that used opposite troop types, if you wanted to, besides Sisters. I could make an all Kroot list, or all FW, or all scouts, or all tac squads.