I disagree that exclusivity is great for both companies. Exclusive agreements may benefit Sprint, but Sanyo is missing out on HUGE sales from Verizon. If they began adding GSM capability to their US handsets it would open up another 70 million potential customers (Cingular/T-mobile).
Also, when certain handsets are available only on Carrier A, the phone manufacturers that server Carrier B may not feel the "heat" to compete and create something better. This, IMHO, stifles creativity and is only bad for the consumer. Not to mention possibly lower handset costs when Sanyo is selling 30 million units instead of 15 million *
*random numbers used to illustrate bulk buying power benefits to consumers.