I never boxed with a rogue (other than my mixed team). But if I had a choice to start one, I would pick which partner (alt) would go with what you are trying to achieve. I do large scale PVP, so I would want a druid to stealth with me. It would let me stealth to get to places and if I wanted to set up a trap, I could just pop my druid out of stealth and park him. Then when a person comes up a gank a lowly druid, I could get him.

I agree with a pally being able to tank, that might be nice. Also, he could judge from 10 yards (while on follow of you), which would stop him, put him in melee and have a life regen ability (judgement of light). Bam, it helps heal him, and you and you won't have to do much to worry about him.

The priest would be nice for fears (to help with defense), anti-magic (dispel), DK's diseases (cure disease), bubble and renew while moving and fort for stam. All seem pretty nice. But he will be sticking out like a sore thumb in PVP. Unless you can really stop someone from ganking the priest, you will constantly be losing a healer.

Also, having someone to dump aggro off onto seems nice. It fits with the playstyle of the class (not to tank mobs). Dumping aggro onto a bear or a pally seems good. You can't really dump aggro onto a priest and have him live through it.

But really, just pick what works for you. Maybe you like and understand pet classes. A hunter's pet can tank for you. It can do AOE to pick up mobs (tenacity pet), it can be healed easily by the hunter and you get free easy (ranged) DPS from the hunter itself. Same mostly with a lock (voidwalker or felguard).

The ones that don't really work is non-healers and melee also. Both really don't work out well.