Holding back on the requests summary doesn't make it more likely that I'll bid on one of those IPHs. It just makes it more likely that I'll go look for a treat or pinch hits elsewhere and be too busy by the time requests are revealed to write a treat. In two of my past five YTs, stories I originally began as treats were eventually posted as fills for PHs, so from my perspective, keeping the requests unrevealed reduces the chance that I'll supply a PH later while not increasing the chance that I'll take an IPH now.