Indents with tabs are bad? I always thought this was a good thing, allowing each user to set their tab spacing.

Languages where indents have semantic meaning tend to prefer spaces. Python style guide [1] explicitly specifies spaces and while Ruby community doesn't have official style guide all their popular unofficial style guides prefer spaces over tabs.

I think the problem is that it's so easy to accidentally mix spaces and tabs that it is more convenient to disable tabs and just use spaces.

[1] http://www.python.org/dev/peps/pep-0008/