I think it's a lovely idea. I use multi-purpose lots all the time but the ability to add a secondary lot type is wonderful and would be so useful. Right now, I work around the limitation by changing lot type during gameplay. I've got a few lots I do this with.
I have my Creek Street, which is a Bowling Alley, Cinema and Restaurant. It's set as a 'bar' most of the time, since that generates NPC bowling teams and the Movie Hangout Stuff TV screen fits the criteria. When I want the group I'm visiting the lot with to have dinner, I change the lot type to 'Restaurant' (I set the menu up when I built the lot) and wait for the host, waiters and chef to arrive (didn't take too long). Once I was done, I changed it back to 'Bar' again.
I also do this with my Up The Amp Music Festival Lot. I leave it set as a 'Karaoke Bar' most of the time but when I want there to be some dancing instead of singing, I switch it to 'Dance Club'. It seems to work well and doesn't have too big an impact when I'm playing, either.
It would make that job significantly easier if we could add a secondary lot type though <3