I think wrongly-branded buses appearing on the wrong routes is bound to be inevitable during the evenings and on Sundays though. When the Silver Arrows were first introduced, they'd interwork at Washington with short M2's / M3's to/from Heworth (through-fares even used to be available!)

Apart from the Northern SPD's, there's not too many non-branded single deck vehicles. Evening/Sunday services are often quicker in journey times and less frequent, which makes interworking with other services more attractive to achieve maximum operational efficiency. It's not going to be practical to have buses laying over at bus stations for 20 minutes at a time just to keep branded buses on their correct route. Using Northern vehicles on 2A/2C/61 could be a possibility, but there's the constraint of not being able to use deckers on 2A/2C, and I don't think passengers on 2A/2C would be appreciative of Northern SPD's appearing every evening/Sunday in the interim until their withdrawal.

Back to the 61; although the PVR is five which qualifies for branding, this is a route I'd much prefer to see become Northern rather than continue being the poor relation to the 60, which has twice seen new buses purchased - Optare Versas in 2009 and the upcoming Streetlite order.