To elaborate on something I said in a comment on here, the theoretical advantage of using a separate filesystem for /boot instead of just putting it on your / filesystem is that your root filesystem isn't limited to environments that GRUB (or the bootloader of your preferences) can understand. This matters (in theory) because GRUB more or less inevitably lags behind the operating system in general; first the code has to be written in and stabilized in the kernel, and then it has to be ported into GRUB.

Whether this an advantage in practice depends on what you want to do with your root filesystem. At the moment, I think that the only major limitation that most people are likely to run into is that GRUB apparently doesn't understand LVM, so this may not be much of an issue. (On the other hand, lots of Linux distributions seem to more or less default to LVM these days.)

People setting up more esoteric environments (or just wanting to be prepared for them) are more likely to run into this, especially if you want to live relatively close to the bleeding edge. (For example, I'm not sure if GRUB supports ext4 yet, which means that if you want to be able to upgrade your root filesystem to ext4 the moment it's supported in your distribution, you're going to want a separate /boot.)

Given the GRUB issues with LVM, I expect that I'll continue using a separate /boot filesystem just on general principles.