Yeah I see where you are coming from but surely there should plenty of single deckers available for the 88 such as Cadets, Versas, Scanias etc. Even if the 88 needed to use an MPD surely it would be better to allocate a cadet to the W5/W6 instead of a decker. It just seems silly using a douible decker for services which are so quiet. I have often seen only 5 or 6 passengers on board from the Galleries to Barmston Court/Columbia. Services M2 and M3 get much better loadings so surely they should be alloacted the deckers if needed.
Problem is if you were to allocate all of the single deckers to service and there was only double deckers spare in the depot and a bus broke down on the 88 you may as well have no buses in the depot as double deckers aren't suitable on the 88, so double deckers are sometimes allocated to other services to make sure they have single deckers spare for the 88.