I can only offer an explanation to why Orochimaru retains his summons:

Perhaps the blood used in a summon jutsu is merely the link between the Summoned Entity and the Summoner? The particular blood lets the Summoned Thing know who is trying to summon it, sort of a proof of authenticity to summon it...

The blood is filled with the chakra of the Summoner, letting it ID the soul of the Summoner. Oro can put his soul into various bodies (he may even be changing the makeup of the possessed body's blood...) and thus his Soul would still be present in the blood of the 'vessel', despite it not being his own.

(though this is directly contradicted by Naruto summoning Gama-bunta, the Frog Boss. The Frog Boss did not know Naruto and was quite pissed to be summoned by him... This suggests that the blood is more like a homing becon in the living world to which the Summoned Entity will be attracted, manifesting at the blood spot)