For one thing, Google Voice sms does not work with shortcodes, so if you want to use any sms service based on a shortcode, Google Voice will not work.
What do people use shortcodes for? Especially with a smartphone in front of them?
Shortcodes are an easy way to type 5 or 6 numbers and send a quick message. I use the google shortcode to get phone numbers for local businesses. It saves a trip to the browser and is an easy way to retrieve information without leaving the "phone" feature of my smart phone.
Some people cannot be convinced that there is any other form of communication available from a cell phone. :)
I believe you still cannot send multi-media texts with Google voice yet.