Random ideas topic
Except for the fact Ichigo actually became a hollow. on page 55 of Volume 8, in the top left panel, Ichigo was shown with a complete mask that showed the odd style eyes hollows have, a slightly open mouth, and a hollow's hole.
Simply because Ichigo exerted his dominance and supressed Hollow Ichigo, even unintentionally, did the regular Ichigo continue to exist, as it showed that Ichigo was a hollow. That is the only reason hollow Ichigo exists, as Ichigo was a hollow.
Edit: and being that Hollow Ichigo is a part of Ichigo, was eventually able to grow strong enough thanks to the Bankai training, and become dominant over Zangetsu and become Zangetsu. When they first met, Zangetsu supplied two swords, the asauchi (the one given to Shinigami not worthy of becoming part of the 13 court guard), and Zangetsu. Hollow Ichigo emerged and grabbed the falling blade, that was still regular colored, which is soon returned to Ichigo. Hollow Ichigo had no sword on his own.
However, when they next met, Hollow Ichigo had become dominant, and had his own blade, a reverse colored Zangetsu. Showing that he had becom Zangetsu as before the 'old man Zangetsu' was always seen standing on his Bankai form zanpakuto.
At least in my opinion anyway.