I fail to see how putting this in would in any way, shape or form effect that you can have access to any demon you want at any time?
You summon your Imp, and you(and it) have access to the imp tree.
Next you summon your void walker (there by causing your imp to go bye-bye) you gain acess to the void walker tree.
If you have no pets, you have access to the last one you summoned, making it possible to set the talents for that one pet that you have to summon and enslave.
Since the talents would only effect their own spells, or give them new ones, quickly changing from one pet to another would have no draw backs. If your imp gets a new buff with his talents, it would drop once it is desummoned.
Eh, was just an idea I had after hearing the woes you guys are having with your untanks.