I pretty much exclusively fly Delta, and have repeatedly see them make announcements about "only two items" AND stop people from boarding until they consolidate. Not every flight, but more often than seeing someone go through with 3+ bags.
Of course, someone could consolidate, then separate while going down the jetway or once they get to their seat.
I'm torn on this one. I do think a passenger should be able to put a bag into the overhead and leave the under seat free.
BUT, I don't have a problem with the FA ASKING if they can put a smaller bag under the seat to leave more room in the overhead for other's bags. IMO, they should not be rude or insistent,
BUT, if you disobey a FA, you could find yourself kicked off the flight. I don't know if that qualifies as "involuntary denied boarding".
@flygirl, any insight?