0.1% of people in the world? That seems a little high. 0.1% of people who describe themselves as *-dev? That seems really low. Also, how do you define "need"? Dtrace isn't just for tracing functions down to the kernel level, and I can imagine all sorts of things that a web dev would find useful in dtrace. Just google "ruby dtrace".
If you do any kind of programming or system administration, you need DTrace (or some lame Linux equivalent). It really pains me to see that people reject it because it's "too advanced" for them, or such nonsense. Dynamic tracing is the one technique that will enrich your life as an IT professional more than absolutely anything else ever did.
And DTrace in particular is really, really easy to use (the Linux alternatives, not so much).
Everyone needs to know all of the things. Just because it's a pain to get now doesn't mean people wouldn't use it if it were there. Remember when monitor mode meant patching drivers and running a weirdo kernel? It's like that.
- If you use Dtrace to troubleshoot a problem it's way beyond your scope of knowledge as a web developer. Dtrace is too deep and very specific to certain problems that you will never see because your regular nginx + php works fine in all cases.
- Not everyone wants and should be good at everything.
Just be good at what your doing well, I'm not saying that one shouldn't be curious but Dtrace isn't "an excuse" to install *BSD as a web dev.