I've just updated my Linux kernel and I wonder if there is any other concern apart from being able to recover if the new version can't boot.
What do you think?
The biggest reason I've found for why you might want an old kernel is because it's a specialty kernel and there's no recent counterpart. I have an "old" -rt kernel because there is no "current" one available mainstream.
Specialty kernels aside, though, as far as I can personally tell or care, old generic ones are obsolete except for recovery purposes. I've never had a compatibility issue between an old program and a new kernel.
No other reasons than to be able to boot should the latest upgrade go wrong. I tend to keep one or two older versions available and kick out the rest.