how likely will the camera lens get dirty because the user was trying to press the volume buttons?

Don't think it will be an issue since the volume buttons are textured.

Sent from my G2 running SOKP
 
In the course of handling any phone, you're liable to touch the camera lens at some point, especially when taking your phone out of your pocket. Just a simple wipe of the lens and you're good to go, so I don't think this will be an issue.
 
I hit the lens on my flex a few times at first, but once you get acclimated to the layout it's a non issue!

SENT VIA MY LG G-FLEX ON SPRINT, POSTED BY ANDROID CENTRALS APP....LIFE IS GOOD✌
 
I don't even clean the lens on the S3 and 4S that I use. It's always covered in smudges, but hasn't really affected photos or anything in real world use.
 
how likely will the camera lens get dirty because the user was trying to press the volume buttons?

This has not been an issue on my G2 which also has the buttons on the back just under the lens. The lens is protected enough from fingerprints.
 
I have a Note 2 and I get my lens dirty all the time and there's even no buttons around it.. It'll depend on how you used to hold your phone. Buttons won't matter much.