Well I also work every now and then in substitute hospitals for a quick buck, but this is exactly the reason why I dont always do this. I like working with colleagues, and also when you substitute somewhere you are always the new person who has to 'prove' themselves, while now I can be myself (which I do value a lot haha)
So for me, wouldnt want to do this always, but for a day here and there, I get it...and it makes you flexible which isn't a bad thing at all. A simple calculation on it...if I do this today, I can spend this tomorrow on x-y-z