What would make it worth two bar slots?
Personally? Nothing. Nothing in this game is worth two slots. The number of abilities is limited enough, and having to double bar abilites cripples build diversity. I am sure some consider it making "interesting choices," but I think it just limits how the class can be played.
I would prefer pets to be reworked into something similar to the NB shade. A duration-based DoT that can't be killed or targeted, but doesnt require baby-sitting to keep alive. Obviously this would require significant reworks to how the pets currently work, as having an on-demand heal tied to a duration based summon is silly.