Shale is pretty poor to begin with but once you get better crystals (usually by levelling above I think 10, the better ones become available in Orzammar) he becomes fairly decent.

I find Leliana fairly useless in my party outside of opening locks, disarming traps (both great XP sources) and interrupting mages.

Morrigan is a great offensive caster. Go for the ice route and then the entropy spells for some great fun. They are mainly disable, not damage (though ice + vulnerability can pack a punch)

Healing mage would be quite useful actually, haven't come across one yet but I heard Wynne is the choice. Still, healing potions are around in ample supply and you can craft them too off the bat.

I find the biggest problem is finding someone who can reliably dish out damage quickly. My main is a S/S warrior but since I mainly raised STR/DEX and neglected CON, he can deal damage and take some, but doesn't excel at either. Massive armor also means slower attack.

I imagine a melee rogue wouldn't be too bad since you can get some top notch light armors at the Warden keep merchant (Inscribed leather) which gives loads of protection at almost no fatigue cost at all and is fairly cheap to purchase. With the dagger bug, you may want to consider sword/shield rogue though or alternatively sword/sword or axe/axe.