Now is it really necessary to update the kernel?
You should use the latest updated kernel.
What are the pros and cons of updating kernel?
There could be exploitable vulnerabilities in the current kernel. There could also be bug-fixes, API changes(less likely), or new features. I understand it's a pain to recompile vbox modules or whatever, but I haven't had a kernel update break something in quite some time.
After updating kernel, should I keep older kernel version?
Keeping the old version let's you roll back the changes if you get a kernel panic in the case of a failed update. Keep it until you have at least tested the new kernel or run it for a week or two...