A USP is fantastic if it actually sells. Ironically, it does sell but it antagonises many of our pax and is unpopular. Again, go onto skytrax and see how unpopular unallocated seating is. That is the most consistent complaint ulyou will find.
As for crew saving seats for off duty crew... Certainly at MAN this isn't a widespread issue as it's more hassle than it is worth. Believe you me most just get pre-boarded. Furthermore, we only get easyjet MAN based crew (people who we know) travelling as PAX once in a blue moon (I.e when they go on hol) because very few crew position on MAN flights. Consequently, you will find that on all but rare circumstances the wheelchair seats will be for wheelchairs because the travelling crew are on first anyway .
Credit where credit is due, crew generally do a great job at getting pax seated and you will find that pax trying to save middle seats irritates crew every bit of much. Certainly many will actively seek a pax travelling alone to occupy that seat. Works a treat!