Plasma are the terminator/marine killing weapons of choice, and thus should always be included, even against things like Orks!. Pretty much half the available armies and more than half the games you will ever play feature either power or terminator armour equivalents. Melta are obviously vastly better than plasma at tank killing, but have only have half the shots of plasma at 12 inches. My plasma vets throw down 7 plasma shots at 12 inches, more than enough to vaporise a marine combat squad!
Regarding the unleashing all your guns Finlay, in my experience, I often choose not to fire the heavy bolters on my LRBT because of wound allocation rules. Say you inflict 5 wounds on 5 marines with your battlecannon, that is 5 dead marines if they are in the open. If you fire the 3 heavy bolters as well, you could inflict say another 4 wounds, which could be allocated to allow the marines chances of survival by stacking two of the savable wounds on one or more models. [EG marine 1 and 2 stack the 4 heavy bolter wounds, while marines 3,4 and 5 take the battle cannon wounds]
If they are in cover though, you might as well fire the heavy bolters.
Depends on your main gun though. If you tanks main gun has the same or worse AP than the heavy bolters, stick bolters on it. EG my Punisher throws down 29 s5 shots a turn.
Rogue psykers can be done cheaply by Flaggies Rufus, they look pretty good as raving looney wizards IMO.
EDIT: Finlay, I hope you dont think I am being harsh or overly critical, I am just trying to convey what has happened to me in the past! Sorry if it looks like it