User blog comment:Uraharattack/lolz NAZI Ichigo/@comment-3246134-20130607221837

i feel this is the correct interpretation: "Zangetsu" knows about his existence and that Ichigo must stay alive for him to exist as well

Knowing Ichigo's reiatsu coming from the Shinigami powers, he knows that the "hollow" part will overtake over eventually, so he acts like a filter to limit the power to avoid an overload on ichigo's power

About the getsuga, "zangetsu" could just drag it from Hollow Ichigo and claim it is his. Why? Remember that Hollow Ichigo is more proficient over using the getsuga, even more than Ichigo

my veredict(imo)

Zangetsu is acting as a Zanpakuto and a limiter to protect ichigo from an overload