A YS isn't just for "filling up" or always even for building towards guarantee. Many pilots YS trips just to increase control of their schedules. Especially in obvious cases where you know you're going to get something, so you preference the least worst something you see at the time. That could result in a favorable report or release time, an opportunity to deviate from a DH, get a nice layover somewhere (maybe even at home), avoid a redeye, get a redeye, easier trip etc. There may be great reasons to YS a trip even if there's zero chance to increase pay/hours. There may be a few cases where YS'ing a trip that starts on a day off is better for you (commute, DH DV opportunity, layover, etc) even when you won't make a minute more pay.