What is more important to your clients:
a) Your convenience?
b) Their (potential) client's impressions?
A major part of a web site's purpose is to foster a brand identity. What would not supporting their browser say to them? If, when Microsoft end support for XP there is a significant shift away from IE8, you might have a case, but for the time being, people will continue to access your sites using IE8, whether you like it or not. You need to support it.