Due to increases in sensitive data being lost they clearly want all possible sources of said data to be encrypted. This may or may not be overklill depending on your opinion but one thing is for sure and that is that it's their decision to make.
If your not happy having your personal computer encrypted (And I know I wouldn't be) the simple solution is don't use it at work, use a work computer. If the requirement covers you checking webmail from a personal computer at home, where you will have access to sensitive data, the solution it to not check your email from home.
If you are required to check email from home and are not happy to have your whole computer encrypted then your employer should provide you with a company laptop which they can do what they want with, encryption and all.