Jeff - I run Mach 3 from a laptop. I think the problem is the word - laptop.I am lucky, in that I got this laptop from my daughter, Jessica. She had bought it prior to going to Universirty, and it cost upwards of £1000 (plus £500 repair warranty) so it was not cheap. It is a full feature Toshiba.
The trouble with many laptops is that, to get the price down, the spec is cut, and many of the functions are shared. The memory is "borrowed" by whichever part of the machine needs it at the time, and the processors tend to be slower.
That being said, I think a laptop in the workshop is superb. It is compact, and at the end of the day you can close it up, so it is dust free. That being said, the screen just went on mine, so I have a spare thin screen attached to it.
By all means buy one and try it - by all means look at the spec. Put in an extra memory chip if possible. If it does not work, there is always Smmoth Stepper, which takes away from the computer a lot of the "donkey work" of running a CNC machine, and leaves the computer free for the real work.