Probably depends on your process. If you have unit tests that cover your code, then continuous integration is worth every bit. I'm assuming that you guys all work on a single module of work as the projects are 2-3 weeks.
I don't think folks will run every test for every one of their commits and continuous integration helps a lot here.
The other reason would be if your project is highly modularized. I've worked in systems where there are lots of modules and a developer wouldn't be functional-testing the entire website before committing. Things might not even compile properly as the other module wouldn't even build because the developer did not checkout the complete code.
I'd recommend continuous integration anyway. With setups like Hudson and Cruisecontrol, it doesn't take a lot of time to set up and pays for itself quickly.
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