I tend to use ground work as a 'mental check' before I get on my horses...even older ones can have days where they just don't want to pay attention, but a little bit of "well fine, you don't want to pay attention, then we'll work harder and longer" wakes 'em up pretty quick. I think properly done ground work has it's place, even with well trained horses, and it's certainly not something I let slide, because what happens if the horse get's injured, or sick and your "stuck" with only ground work until the horse is well enough to ride?
However, that said, I don't spend "hours" on end, doing ground work, either...quick reminders of proper lunging, round penning, etc, are all that's needed to keep the horse fresh in it, unless there is a spot that get mucky, then you work through it, and move on. I probably spend about 5-10 minutes out of 3-4 hours doing ground work with my mare now...before it was more, simply because she needed it more.
When I am working with a young horse who is untrained, I make sure he knows how to move on a lunge line, and round pen, make sure he can bend each way, can yield his hip and shoulders, and then once he is desensitized to the saddle, I get up and we start working undersaddle.
It can be different with horses like my current mare, who have a less than desireable history, but even with her, I taught her the above mentioned 'basics' and was on her in less than two weeks after I got her. From there we've gotten plenty of wet blankets, and she has settled down ALOT from her first few rides, although she never bucked, or otherwise went bronc on me, either. Sometimes undoing damage done by bad training, is sooooo much harder than working with a colt who's had NO training at all.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."