PhDs often make the worst possible developers. People with PhDs (especially in Mathematics) are often lacking the most critical characteristic of a good developer; they don't know why they do things. Being good at math requires being able to solve problems whose solutions often do not have any practical uses or whose practical uses may not be clear. This is very different from how software problems should be approach…
> The PhD diploma itself doesn't have much practical use in financial terms; it's not a good investment to get a PhD for purely practical reasons.
I don't know if you say this because you have don't have one, or haven't worked with many (not just one) PhDs?
In the UK at least, your starting salary will be significantly higher than just a plain degree, and it also opens up a broader job market (in the City, at least, and aerospace and defense) where PhDs can be a requirement in some jobs because (junk) degrees are now ten-a-penny.
You learn writing skills, and analytical skills both of which are financially useful and practical.
I get the feel that what you're implying is that PhDs are less imaginative and perhaps less likely to be entrepreneurial. In that case, you may have a point.