The Linux kernel comes with thousands of different drivers for all kinds of devices which adds weight and such that even slows bootup times. For the home user, sure, default works best.

Why use a kernel with bluetooth modules, wifi modules applied when the actual server has none of that hardware? Not forgetting added security risks. It's all clutter and bloat that's not needed at all.

Do you really need to support a 1998 3Com IPX network card on your production server?

It's about understanding the system as a whole not just the userland or /etc/. Sysctls parameters and fine tuning the cogs that drive the system; the Linux IP stack is abystmal without tuning.

The default kernel works, but your not going to get near best performance without fine tuning and minimalizing the kernel. It's not a bragging excercise, it's a real excercise to ensure the system is secure and to ensure smooth operatation. You'll find these excerised in enterprises where such performance are a requirement.

Knowing how to compile your kernel is key piece of important knowledge to know and you'll find that someone with that knowledge will end up getting hired than someone without.

The saying is true: SysAdmins treat servers like pets; developers treat servers like cattle.