I myself have done it a couple of times thinking its a few hundred extra €€€ in the back pocket, then roster friday arrives and low and behold my 4 days and a standbys turned into 3 days and 2 standbys! both times
A mate of mine in a smaller base was was having the same experience with rostering asking him to work 6 days as they were always short F/O's in the summer and he also kept losing a day to a standby that was originally a flying day.
Why they do this is beyond me, maybe it is down to max hours or something, but if thats the case there really is no point in taking the day as you will just end up with the same hours/money but have a much worse lifestyle because of it
I will not be accepting any days off working for crewing at all now, still don't mind to do a swap for another pilot as this is helping out a college and changes nothing other than a favour which goes both ways if needed and not forgot unlike with rostering.