I am a consultant who sometimes writes code to do certain useful things as part of larger systems (parts of which may be commercial or GPL) but my clients always try to impose terms in their contracts with me which say that anything I develop immediately becomes theirs, which limits my ability to use it in my next project. I can to some extent circumvent this if I find a way to publish the work, or some essential part of it, under an MIT license. I’m never going to make money off of my code directly; at best it’s middleware, and my competitors don’t use the same stack, so I’m not giving them any real advantage… I don’t see how I’m sabotaging myself in this situation; if anything the MIT license is a way of securing my freedom and it benefits my future customers as well since I don’t have to rebuild from scratch every time.
The problem is, that would limit my own option to make a version of the software and sell it under a more limited license in the future. Whomever I sell it to then has the right to go ahead and redistribute it, competing with me. Sure, my current, highly niche code already carries that risk, but the MIT license doesn’t stop me from releasing a modified version I may write that is more valuable as software, and then protecting that release with other licensing terms.