I've been on three different teams - 201, 830, and now 121. Two of them (201 and 121) never have/had a second robot. One of them (830) built a second robot every single year.
The two biggest issues are cost of materials, and additional fabrication time. First, you have to buy every single part in duplicate, which can add up, even if you use (like we did) relatively inexpensive parts. If you have a decent budget, or work really hard to raise the money, it can be done, though.
The far bigger concern, though, is the amount of time and effort that it takes to build everything twice. You do get some efficiency and savings from doing the same thing twice, but it still takes about 75-80% of the man-hours to build the second 'bot. There are also a lot of repetitive tasks (16 pillow blocks for drive shafts!) that can be frustrating. Finally, the hardest thing ever is motivating your team to finish both robots by ship day. Two of the three years that I did it, the practice robot didn't get fully finished until after ship day. We just didn't have time to finish both in time to ship one, and once ship day was over, nobody wanted to put in time to finish the second!
This also has the insidious effect of making people think "hey, we don't need to finish the robot until right before ship - we have a practice 'bot to do driver practice". Needing that last few days or week to practice gives a great motivation to finish early. Plus, your programmers will thank you for that time
All this being said, it's been a lifesaver at times, and a handy thing to have at others. You get a chance to practice how to make all your parts before they go on the real thing, drivers who never got to drive their machine before ship day can get much-needed practice, and when something breaks at the end of your first regional, you have a copy back home to look at and get ideas for how to fix it (or test out parts you make in the Fix-it Window!)
So, my conclusion - there are some really good reasons for and against building a practice robot. If you've got lots of time, money, and manpower, you can really get some benefit from it. Otherwise, it can become a drain on all three. I may miss the benefits this year, but I think I'm glad I don't have to deal with the downsides, either.