I recently purchased a very inexpensive and very small set of powered speakers that are receiving a certain amount of attention for a portable down market solution in some applications.
I could clearly hear some resonances, and came to the following conclusions:
1. Plugging the ports lost about 1/3 to 1/2 octave of the lower end, made the speakers a little less efficient, but made huge improvements in every other regard. (Reminder-this is very down market gear!)
2. Some of the resonances I heard were also heard on "real" and much more expensive monitors (even sealed cabinet ones)-but they seemed less obvious on these other monitors as the resonances were broader and thus a little less identifiable on these other monitors. It also seems that in my own case, the resonances were a part of the program material and were simply handled differently by different speakers. OTH, there are many speakers than bring their own resonances to any program material.
3. The position of small monitors makes a huge difference. Moving them a few inches from the lip of a surface or other little moves can change everything.
4. Ports can make a speakers behave quite differently at different SPL's, so that has to be taken into account as well. Generally, I find I need to play ported speakers a little louder than I'd like.
The advantages/disadvantages to plugging ports seem to be very situationally specific.
And a reminder of the caveat in my own case-the speakers were down market and were not intended for any sort of serious monitoring.
You could avoid the whole issue by getting sealed cabinet speakers....you'd have fewer options to entertain!
Or keep what you have and use them both ways. Why not?