Just to make sure that everybody is on the same page -- this thread is referring to the NATIONAL event, not REGIONAL events.
If you're at the National event, chances are your robot is working fairly decently
and that you know what to do on the field. That said, I definately believe that a practice match is still necessary to work out any problems that might have occured during shipping or whatnot. 1-2 practice matches is a reasonable maximum. 3 is probably too many.
I agree 100% with the idea to create a partial field for practice use. Especially if autonomous mode becomes more prevalent (as I suspect it might), it will be very useful to have this area available for fine tuning. It would also be VERY nice to be able to show off your bot for judges on this practice field. Attempting to demonstrate features in the pits has never been an effective way for showcasing our weeks of hard work.
-- My 2 cents.