Why would you ever use crop mode?

There are a few advantages to using crop mode over a teleconverter.

[@johnalexander7831]

Cheers Joel, a very good summary of the benefits of crop mode. I might give it a go after this.

[@danielbottner7700]

Autofocus deteriorates due to less light reaching the sensors.
I use crop mode to increase depth of field & reduce file size to speed up image processing.