(fyi; it's OS X, not OS/X)
macminicolo.net has been around since shortly after the original Mac Mini in 2005, so there are people that deploy on OS X, but it's not really cost effective. The XServe line was Apple server, but they discontinued that back in 2011, so it's likely that Apple doesn't even use OS X in their datacenters, opting more to use more traditional servers with a supported OS. Apple's draconian licensing rules for OS X prohibit wider distribution (read about the Psystar case) anyway, so there's not really a wider market, especially compared to the licensing cost of Linux.
As far as init, OS X uses launchd which has its own foibles (XML everywhere), but it works. The app store is sort of a 'package manager', though it's not really designed as one. What you're really looking for is Homebrew which has all the unixy/webdev tools you should need. Mysql and sqlite work fine on OS X, and for the built-in 'web sharing', Apple actually runs Apache under the hood. The network stack runs fine. It isn't great if you've trying to maximize throughput on 10-Gigabit ethernet, but that's hard anyway.
People use it on development machines because Apple makes sexy laptops with sufficiently big batteries to last all day on a single charge. Because Apple controls the hardware and the software, their laptops run OS X really well.
You can run Linux on a Macbook Air/Pro, but at the end of the day, the Unix underpinnings of OS X are good enough, especially for webdev. Install brew, do 'brew install ruby/python/nodejs/whatever', and you're ready to go. Not sure how many brain cells it takes to remember 'brew install $pacakge'.
However, if you find yourself doing lower level systems work where the difference between a .kext or .ko file is huge, or you have your own opinions about systemd, influenced by the amount of time you spend digging around in systems, then by all means, use Linux as your primary OS, but webdev isn't devops.