They're probably not 100% satisfied with McCarron at receiver, but they don't have a lot of depth - even after Edelman gets back from his involuntary vacay - and they don't have much young, developmental depth.
I suspect that the Pats will be regularly having WRs in for tryouts for a good part of the season, and probably some shuffling through the PS.
BTW - the advantage to putting Hansen and Darboh directly onto the 53-man from waivers (rather than on the PS), it that it preserves their existing contracts. Both are currently signed through 2020. If one works out, then they already have him under control for three seasons. If, instead, the Pats had put them on the PS, they'd effectively be free agents immediately.