Sure, if you have a legal carryon sized bag (and just that bag plus "personal item"), it doesn't make sense to pay to check it ever. I did that on a short trip recently. When United made an announcement about those in group 6 and 7 (I was 4) most likely needing to check; I immediately went to the podium and offered to check mine. I was rewarded for what I considered a kind act to those behind me by my bag coming out very quickly at SFO.

If you want the space that a larger checkin-legal bag provides (and we do most of the time), then paying to check it of course makes sense, as you're unlikely to get it through both security and to the gate without an agent spotting it (I'm not talking about a 1/2" over bag, but a true 24" or bigger suitcase).