The main reason being a lack of support for MMS on Google Voice.
I use my Google Voice for professional purposes mostly as well as situations such as online purchases (so I can block future marketing calls if necessary). I've kept the other number because it's the number I've had since I first got a cell phone. All of my personal contacts have that number and it's too much of a hassle to explain to my family to email me photos rather than text them to me.
For outgoing calls I have my phone prompt me each call as to whether I'd like to use my phone's number or my Voice number to make the call. It's an easily changed setting to have one number or the other handle all outgoing calls without prompting.
This buggest advantage is the Visual Voicemail. The transcription isn't perfect but it's good enough to get an idea of who called and about what without having to listen to a message. If need be, it's also much easier to listen to the actual messages.