Let me offer a slightly different point of view. I am mainly an end-user analyst and even though I have been caught up in the ownership issue many times, it can be helpful.
I am given read-write access to a single database, which I share with other people.
From a DBA point of view, the questions - who's table is this? who's hogging the space? are very easily answered with ownership tags - but can also be handled with naming conventions (but who adheres to that?).
For awhile, we analysts had everything as dbo, but this too is a nightmare. Import/Export defaults to a non-dbo name, as do other wizards. This made matters worse. Since everyone assumed dbo ownership, when it didn't work, it was tough to debug. In this environment, I had tables dropped and tons of work erased by others having full access to "my" tables with dbo ownership.
We eventually left the dbo world for the analyst database and the this-is-mine this-is-yours worked reasonably well since everyone knew that it existed and permissions had to be granted.
To me, the real question is, if this is truly a bad practice, then what steps should a DBA take to ensure that all wizards and gadgets conform to a universal ownership standard?