Pros
Butts in seat contract gives loads of opportunities to perform in many different skill sets. I've been switched to jobs that gave me exposures to areas I would have never had opportunity to work in prior. Those allowed me to use these experiences as stepping stones to work for far more at money other companies.
Cons
Continued "greening" to bring down labor rates. In my observation the upside is if the employee is cheap then the company will just about any employee. This is not so with the highly compensated (older) employees who need to step lightly. If you're a SME you may be compensated as a non-specialist. They like young people with fresh knowledge of the latest technologies, however they will cap their pay on many contracts so that these persons could be shockingly under-compensated.