It doesn't bother me that much, mainly because I personally dislike Matt Serra's attitude, but you do off an interesting take on the situation.
The interim title fight is most likely to just validate this as the main event. Serra/Hughes was going to be the main event and it included a title, by keeping it as a title fight (whether interim or not) the match remains relevant to casual fans.
I always thought Silva/Liddell should have been bumped to the main event, but GSP and Hughes will be a good fight and with title ramifications it becomes an even more exciting match.
In summary: My guess is the fight was made an interim title fight in order to make it as relevant as the Hughes/Serra fight was. Everyone gets more excited for fights when a strap is involved, especially casuals.