For most quint system players the downside would be the extra weight. You still wouldn't be able to play in any key but with the right accidentals at the bottom of the treble keyboard you'd have a useful range for music from the British Isles at least. It's a value judgement - is it worth giving up the lightness of the 2 row box to gain the ability to play in a couple of keys you'll rarly use? A 2 1/2 row box might make more sense.

For 3 row a C#DG would be more useful.