It is really pretty trivial to add aptitude back if you want it - 'apt-get install aptitude' is not hard for those who can handle aptitude in the first place. Also, the mention of the server installation shows that whoever said it hadn't actually checked the maverick server installation - it's still installed by default on servers (due to its optional use for interactive package selection during server installations).
Regarding upgrades, aptitude has a different dependency resolver which handles things quite differently from apt-get, and the fact is that we simply do not routinely test that dependency resolver. I'd rather encourage people to report dependency resolution problems in the tools that we use by default (apt-get, update-manager, the release upgrader) rather than retaining and encouraging the use of a forked toolset.
The base system is constantly tight on space, and everyone wants a piece of that space. I'm afraid we're going to have to continue to be pretty brutal about what we include there. But, as mentioned, aptitude is of course not being removed from the Ubuntu repository as a whole.