I understand that it may be wise to keep the old kernel around so the system can be booted in case there is a hardware incompatibility or breakage in the new release, but that justifies only one additional kernel. Ubuntu keeps those kernels sitting there until you `apt-get autoremove`, and that means that unless you're running that command routinely, the boot partition is going to fill up at some point, no matter how big you make it.

This is especially a problem for people who use the unattended-upgrades package. I've autoremoved and had it clean up almost a gig of old kernel images before.