Considering that I might have more than 1 team - what's the best known way of boosting your alts?

I remember using my main (a pally) to boost my team back in TBC, by doing instances and leaving the team parked and the pally killing all the mobs. I guess something similar could be done with for instance a mage.

Is that the most efficient way of boosting alts? Does it even work in vanilla?