Does ECC really make a difference in practice? Is it even worth the price? I don't have ECC in my iMac and I'm sure it'd do quite well if I used it as a web server...

It depends on your data, if you don't mind single bit errors in your data non-ECC is fine. If your data has to be perfect then you probably need ECC.

Do you run your iMac 24/7/365 and have customer data on with such data being frequently accessed?

ECC exists to prevent data corruption so that you don't have to restart your server.

Since I imagine you restart you iMac near daily, not having ECC isn't a problem.

I actually never restart my iMac. Once every 3-4 months I'd say. And not because it crashed, because I need to for software updates.

The value of ECC memory has little to do with how long your computer runs or how frequently data is accessed. Restarting a computer won't prevent the problems that ECC corrects. ECC memory is designed to detect and repair memory corruption caused by electrical/magnetic interference or problems with the memory hardware.

That kind of blows my mind. No ECC by default? Is it really a server with no ECC?