I suppose, thinking about it if I were in that position, I'd probably feel under less external pressure if there wasn't a job riding on the end of it, and I'd have more of an incentive to push myself to prepare better if I knew I'd paid out of my own pocket for the tests.
Additionally, I assume if you pay for the tests you do them all regardless of if you fail one of them, whereas with a job application typically if you fail one they don't give you the opportunity to sit the rest. That way if you say, only fail the dots, you're only having to come back and resit that one test, not that one plus all the stage 2 ones etc.
I'd possibly consider it if I had a bit of spare money come in. It doesn't harm to have a pass sitting there on file in case you need a job in the next few years and it's one less thing to worry about during the application.