So of course people thought that when they changed jobs, cable companies, or whatever... they needed to create a new Apple ID with their new E-mail address. This was reinforced when Apple further stupidified their policy by requiring your ID to be a WORKING E-mail address (originally it didn't actually have to work).
After the outcry over people's App Store and other purchases being scattered across multiple IDs, Apple finally publicly and huffily declared that they weren't going to fix the problem they created by letting people consolidate accounts.
The moral: Don't force people to use E-mail addresses as user IDs. It's stupid on several levels.