The usefulness of the 17" is to be able to have two different application screens next to each other and see the whole page. Also the Electronic Medical Record I am using will automatically optimize for high resolution screens which I don't have. Generally I use the computer on the couch so I 24" screen doesn't work well for me.
Have you try using Spaces to match your needs? I just make the app take up full screen and switch space to other app that's full screen. Usually works for me on 13" MB.
If you got the money to burn, then get the 17". You probably can get some money back by selling your current MBP.