Think of plugging a port as a tuning option. The options lets achieve good bass in a variety of situations compared to a speaker that did not have a port.
@noromance is correct, you ARE altering it's sonic signature so that it sounds better in whatever location you place it in.
Speakers aren't golden cows to be worshiped and rotate our lives around them. If you need a speaker on a bookshelf and it sounds better plugged then do that.