Why can't you buy your own devices? I live in the UK and have always bought my own devices, it often works out at about half the price over the term of the contract.
Verizon (and most of US) uses a competing "standard" called CDMA, where your identity is coupled to your phone. Which means you have to involve your phone company in order to switch phone.
Verizon and Sprint use CDMA. AT&T and T-Mobile use GSM. The frequencies that T-Mobile uses are non-standard, IIRC.
AT&T => 107.3M
Verizon => 116.8M
T-Mobile => 44M
Sprint => 64.3M
Anyhow my point is I have no problems adding a personal device to my plan anytime - many times.
And that's a pretty good reason not to go for Verizon. It's not that you don't have a choice.
I am with t-mobile ever since I remember, just because I refuse to give my money to AT&T or Verizon (and also to Sprint as they are CDMA).
It's your money, and it's your decision.