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.
I seriously doubt it would make much difference for deplaning - people already stand in the isle with their luggage out of the compartment long before the doors open.
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.
I agree that it wouldn't cause the doors to open any sooner for deplaning, but once they did, people could just ... leave. The people standing in the aisle with their luggage out of the compartment are the ones who started in the aisle seats. People in the other seats need to get out, reach up, pull down, get organized. Sometimes they have to salmon their way back from their actual seat to the compartment several rows behind them because that's where they had to stow their carry-on because the people seated in row 30 put their bag over row 16 when they boarded[1].
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.
