I'll echo insanerandomnes' point here, from a different point of view. I've been playing for some 4-5 months now, mostly as a Sci/Sci, and only now am I reaching the full potential of that combination. It takes a lot of experimenting and research (and a respec, in my case) to get the full effectiveness out of sci/sci, but when you do, it makes life a hell of a lot easier for your team, in (elite) STF's, fleet events and to a certain extent pvp.
To be honest, when I'm on an alt, I'm often sorely aware of the lack of crowd control when I pug STF's in a team without a good Science captain.
Also, a good Sci/Sci build will inevitably allow for some support too, as a full-aux Hazard Emitters and/or TSS will save people's bacon, at times, and do not share cooldowns with Extend Shields or EPtS. That they recently removed the shared cooldown between polarise hull and HE also helps, as this allows for some serious damage resistance stacking on Sci ships, without disabling your hazard emitters.
Also: it's just fun to completely drop a Borg cube's shields using Tyken's Rift, target shields and polarons.