Better to fly about the place picking off the weak with your BBoS and landing to keep up that kind of work against backfield support units than gun for something that can actually fight back.
I dunno, though: against the weak you have the rest of your army. CSM really struggle against such hard-hitters as Wraithknights and the like, since their fancy overpriced infantry that suck up all their uniqueness die just as easily as normal marines (only 2x the cost per mini!) against such enemies. Riptides, Dreadknights, Terminators, Centurions, special characters... enemy Walkers are a weakness, admittedly, but that's why I pack melta on all my bikes.
Your "assault 10 Tac marines in cover" scenario has piqued my interest. So, Mathhammer, engage! Assuming that you don't assault the marines on the end of the squad, or something, so that all 10 of them can pile in and hit with their krak grenades... well, they're hitting on 5+, wounding on 3+, and getting saved by armor on 3+, so barring you having Iron Arm or something, 10 krak attacks equate out to about .72 unsaved wounds on average. "Usually oneish wounds" seems bearable to me--then you strike back, kill more than half the squad*, leave a few survivors so they can't shoot you during your turn, then finish them off during their turn. Your 265-350 point assault shagnasty has just cleared a 220 point Tac squad for 1 of its 4 wounds, at no diminishment of combat output.
*assuming no asshole of a "1" cropping up for your extra attacks.
...ok, this has been a fun avenue of thought to run down, but not a productive one. *ahem*
Infernal Tetrad, ho!