Apart from the minor hassle of ensuring you take them on/off in the right order (which seems to be a unique Hasselblad problem, no other camera had such issues) the real hassle is that if you are shooting portraits with a tube unless you are on a tripod and/or a fairly fixed setting like sitting them on a chair you will most likely end up hitting the focus limit with/without the tube. This really breaks the flow of shooting and you either put the tube on/off all the time or you don't use it at all.
Of course if you are shooting static objects/macro then this is not much of an issue but the OP did mention portraits specifically.
Of course if you are shooting static objects/macro then this is not much of an issue but the OP did mention portraits specifically.