You're assuming you have access to cron on the machine that code is running. Lots and lots of websites are running on shared hosting, where there's no such thing as cron access.
Edit: Why am I not surprised to learn that you work for a hosting provider? I'm not trying to pick on you, I've just noticed a trend that makes little sense to me.
I run multiple servers, at this minute there's 8 web servers online and 1 mysql server. Each server has a different purpose, some are long term and some are short term (taken offline after purpose served). Having to manage the cron jobs running on each server is a pain, having 8 servers all running different things is fine if you have the time and want to save money, but for me I'd rather pay $20 a month and outsource it to someone else. It's the same reason I use Postmark (http://postmarkapp.com) for my email. I could manage email myself but I value my time, same deal here. Same reason I use google apps for personal email, same reason I use Linode for servers instead of buying hardware and colocating.
Time is my biggest constraint not money, so spending an hour to save $20 isn't worth it.
Agreed, that's why you would use a single, cheap Atom box sitting under your desk managing all of your cron jobs.
>spending an hour to save $20 isn't worth it
This isn't saving $20, it is saving $240 per year, so having your own hardware is break-even for year one. Year two is essentially free.
>it's the same reason I use Postmark [...] for my email
Email is an entirely different beast, dealing lots of config files, blacklists, and a kludge of MTAs/ MUAs. I completely understand outsourcing that piece of operations.
>instead of buying hardware and colocating
There is no reason such operations need to be run from a datacenter/ colo. Any office/ business connection should be adequate to run cron jobs.
p.s. Flattered you care ;)
If there was a place to learn everything there is to know about hosting a site yourself, then that would help. Even better if it was easily accessible, trustworthy, and simple to understand. And more so if it displayed proof that you only need minutes to diagnose and fix the most common problems.