I think the practice bot may even be encouraged by FIRST. They
do allow you to bring in 30 lbs of replacement parts to competition. Do they really expect you to bring in 30 lbs for spares? I think that building a second robot is an important part of FIRST, and it is fair because every team at a competition has the same amount of time to practice between bag day and the competition.
I know some teams may not have the resources, but all it takes is hard work. Ask your sponsors if you need to, explain the situation, and explain how beneficial it is to have a second robot, even if it's made of scrap metal.
Also, building and working with a practice robot makes build season seem longer, which is good because build season is fun

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Vivek

Treasurer, Senior Build Officer, Saints Robotics 1899
Seattle Olympic Regional Chairman's Award 2011
Seattle Olympic Regional Winner 2011 (Thanks 2990 & 3393)
Seattle Olympic Regional Best Website Award 2011