I like the idea of the padlock as then the users get the choice of how secure the lock is, also subconsciously they think the product is cheaper.
With regards to carrying the key around, I would expect the owner to leave the key at home, if it was a combi lock I'd also expect they would be as likely to remember that as they are to remember their radio code so no doubt would have it written on a slip of paper at home, afterall you don't access your OBDII port every day .... unless you have a really temperamental car.....