One reason that I would value is that it would speed up boarding and deplaning. As you correctly point out, the prevailing overhead luggage system provides a benefit to some travelers, potentially to the detriment of others. It's a tradeoff.
For boarding it might speed things up but often boarding is done before all checked luggage is loaded so it will probably not let you take off faster either.
Similarly, for takeoff I agree that it wouldn't necessarily save time, net. But it would help with the frustration of standing in line, backed up on the jetway while everyone is struggling with setting up the initial conditions for the deplaning scenario I described above. At least people could get seated sooner and be comfortable for longer while they're waiting for takeoff.
[1] Based on a true story.