Hmm...I assume those people mean one driver, and for regular loud speakers I'd agree it wouldn't be good. But with headphones and especially in-ear-montiors you have the driver right at your ear instead of feet away. This allows you to use only one driver for the entire frequency range because you no longer need to carry sound that distance.
With loud speakers the problem comes from the fact that drivers that can produce bass need to be rather larger to pressurize a room, 6.5-8 inch cones on up. The larger the room, the bigger your cone needs to be, the more powerful it needs to be, or the more subs you need. Unfortunately you can't move those cones fast enough to produce high frequencies. So either you have smaller drivers that can play highs without the bass, or you have larger drivers that can play bass but not the high end.
But, with headphones, the distance from driver to eardrum is anywhere from 3/4's of an inch to about 2 inches and the volume of air that needs to be pressurized for bass is virtually nonexistent as compared to a room. This means that a small driver is can produce effective bass and since it's so small, it won't have any problems producing high frequencies.
So one driver can play the entire frequency range. On top of that, you will have a more cohesive sound up and down that range since it's only one driver. One big draw back is that you can't really eq it to a flat response as the shape of the ear plays a significant part now.
Given your budget, headphones would be a better option for the purpose of music production. For instance, the speakers you listed only play down to 90Hz before dropping off. For reference piano's can play down to 27hz, tuba and double bass down to low 40's, cello 60's, bass guitar usually starts in the 30's or 40's.