Isn't sprint already the carrier for virgin mobile?
Seems to me they are only getting the existing customer base and brand...
Yes, Sprint was already the carrier for Virgin Mobile as well as Boost, which it already owns. Sprint simply bought it's only competitor to Boost (though it was really a sharing arrangement since VM used Sprint's network). Sprint now owns all of the Virtual Network Operators in America with Boost and Virgin Mobile, so this isn't really much of a big deal.
And don't forget about Virgin America, the greatest airline on the planet, when discussing Virgin products and services in America.