Let the hive mind of Engadget get that for you.
"I'm in the market for a new phone and money isn't a limitation. I'm also not partial to any particular US carrier, but here are some of the features I'd like to have: WiFi, GPS, good coverage in lots of places, push Gmail (a must!), physical keyboard (a must!), a touchscreen, decent battery life and a relatively slim body. And please, nothing that has a fruit logo on it. No offense to the fruit fans, though. Thanks!"
There is a huge benefit of locking these phones. First, they subsidize every unit they sell by at least $150. So by locking the phone to only work on their network, they prevent you from buying the phone then moving to another carrier. This is especially important for Pre-Paid phones which also carry a subsidy and therefore protect the carrier's investment in the devices.
Verizon allows any phone with a range of ESNs to be activated on their network, so if there is a manufacturer for Sprint and Verizon you will likely be able to activate any sprint phone on Verizon. Same goes for US Cellular, Leap and MetroPCS. Those companies love when people bring their own phone as the cost of aquiring that subscriber just went down by at least $150, so they profit from you. Plus they will lock you into a two year contract (most of them) and what did you get for that???? Nothing.
What people don't realize if that if the carriers cannot protect these phones subsidy, then they will just stop subsidizing the phones and you won't get any more at $29 a piece.