Personally speaking I'd get bored with always having to connect my mobile phone via a USB everytime I wanted to send an SMS/MMS - you're effectively using the mobile as a media gateway to the mobile phone network.
Using a media gateway partner instead means you don't have to worry about 3G/GPRS being available to sending phone, you just need an internet connection. A nice feature is the HLR locator, which can tell you if the receiving mobile number is registered to a service provider. i.e. you're not sending out blind MMS and paying for a message that will never be received.
The partner does away with the mobile phone pushing out your PC authored message out through their 'media gateway' to the mobile network on your behalf. OK you still have to pay but its convenient.
Consider looking at routomessaging.com , intellisms.co.uk , txtlocal.co.uk
they might offer you some code on basis that they could have 1st refusal on enhancements that you may develop ...............
good luck
Jim