I think I've heard of some DIY CNC machines using full extension bearing guided runners, but, like you, I would have thought there'd be far too much play. I suppose it would depend on the quality of the runners.
I have seen people use full extension drawer slides for X-Y tables. For router mortising machines and such.
I personally haven't used them in any such way.
Not sure I would use draws runners for anything requiring precision. RS used to to do some bearing tracks but cant find them now. Some interesting bits on ebay http://www.ebay.co.uk/bhp/linear-slide
I used them for a home made router lift. Yes there is play, so you need to use them in conjunction with another method such as sliding dovetail or dado.
The other major issue is the ease with which sawdust can get in them, so you need to try and ensure the runners stay covered