I guess Air Malta are just making good use of it until the lease runs out, Who would,nt.
I've never flown on Britishjet so don't know how comparable they are, so I'm only talking theoretically, but a couple of reasons why might not be a good idea
1. if they don't have any aircraft of a similar type
- servicing problems
- airplane steps
however as long as used on the same route as before, should be OK
2. customers - is the aircraft up to Air Malta quality standard ? - in the case you mentioned, what did the passengers think ? would you have minded, given you had paid to fly with Air Malta ?