They blow all the treble past your knees, which makes it very hard to hear what's really coming off the cab. Treble is much more directional than bass frequencies (look at the graphs that come with studio monitors to get in idea of what I mean.)
More than a few degrees to the side, the treble content falls off a cliff. So a straight cab may sound warmer/thicker to the guitar player standing near it, but to someone further way (and thus more in line with the drivers) or to someone standing lower than the stage floor so their head is more inline with the speakers, the sound is much brighter than what the guitar player hears.
So if the guitarist (me) is trying to dial in the amp, I've likely made it too bright and shrill for anyone else if I'm using a straight cab.
IME.