Deleting slightly off-topic posts is a bad idea.
When I complement someone on their car in a thread not related to their car, there is no need to delete my comment. If you want a real community, then there should be candid exchanges between members.
Likewise, when you do delete something that someone took the time to post, it would be courteous to tell said poster via PM why the post was deleted.
Thanks for being moderators though - I know it is a thankless and time consuming job. Case in point right here : )