My first VA was a Science officer in a cruiser. Worked just fine for me, up to the triple failure I had with Khitomer Accord (my own fault, since I'd never done an STF, and my reaction times were off). I dunno, I think it's mostly preference, play style, and skill selection.
I found that, at least in PvE, it's good for cancelling out that "methogentic" (or whatever it's called) sheild those minor races you encounter in the cluster explores (Vito'D, Scoodians, etc.) use on their cruisers. Just save your other buffs until you can cancel that ability, then unleash all heck on 'em.
As for a sci officer in a cruiser, I agree with the other posters: while a sci ship compliments a sci officer better, you shouldn't feel as if you're limited to a certain class of ship because of your career choice. My Gorn engineer pilots Varanus Fleet Support Vessel, which is a science ship (equivilent to the DSSV on the Fed side). It's just a matter of how you spec your skills.
SNB is a great sign how different PvE and PvP are. In PvP, it's a killer, often the key requirement to disable an enemy team.
In PvE.... Yes, i think it's pretty much only good for the metaphasic shields of some of those NCPs.
If PvE enemies would not simply get hit point and damage bonuses as they scale up, but would get a larger set of defensive and offensive powers, a power like SNB would be worth far more, and NPCs would also be more interesting to fight.