Success is iterative and progress is incremental
There have already been quite a few posts that sum up my thoughts on why I think that having stop-build day is important, but I wanted to comment on the classic refrain of "some (maybe a lot) of teams have more resources than us so we can't do X like they can."
Are there teams with way more resources than you? That's a YES for most teams. Is it true that the team with more resources can do more things and is more likely to be successful*? YEP. But that team over there (you know the one I'm talking about, the one that always wins in your region and probably has half-a-dozen practice robots that can fly and shoot 30 discs/second

) had, at some point, as many (maybe a little more, maybe a little less) resources as you have now.
They didn't get there overnight. They made a concerted, year-round effort to constantly challenge and improve themselves, incrementally year after year. I will use our pit area as an example because it is illustrative and perhaps less blood-pressure-raising than the usual discussion of robots.
In 2008 we had:
-A robot cart that had wheels. Kinda...
In 2009 we had:
-A very nice and painted robot cart that definitely had wheels.
In 2010 we had:
-A very nice and painted robot cart that definitely had wheels.
-3 very nice tool carts.
In 2011 and 2012 we had:
-A very nice and painted robot cart that definitely had wheels.
-3 very nice tool carts.
-A small Community Outreach display.
In 2013 we will have:
-A very nice and painted robot cart that definitely had wheels.
-3 very nice tool carts.
-A sweet battery charging cart.
-A complete pit area with integrated displays, signs, shelving, and general awesomeness
We started in 2008 with no engineering support, no support from the BOE (we actually got penalized for missing school to go to our first competition), 11 kids, 4 mentors, and not much more than a NASA grant. Now we have engineering mentors and companies, FANTASTIC support from the BOE (THANK YOU for the soon-to-be practice facility

), 36 kids, almost as many mentors, and a budget exceeding $40,000. We didn't get here overnight, nor did any other team. Our journey toward getting an identical practice robot "fully" functional 2 weeks before bag day was just as incremental as our journey toward having an awesome pit. We aren't in the best geographic or demographic location for running a robotics team (something we are working on) but we also aren't in the worst. It will take some teams more time to get to same point as us, some teams less. Some teams will be able to go further than us, some not quite as far. But EVERY team can improve. EVERY team can work year-round to build up the funding, the spare parts, the team-experience necessary to build a practice robot (or something that can suit their needs just as well) but it won't come easy and it won't come fast. Just always keep in mind that every team started from somewhere.
Apologies for the long post,
-Luke
*I'm defining "success" here to refer to robot performance, awards, etc. for the purposes of this post. My personal definition of "success" has much more to do with inspiring your students and your community and I think in that sense every team is already "successful" just by participating in FIRST.