Sure, you could do that if you had unlimited developer time and people using old browsers are an important market. but that isn't always reality, you've got to choose where to focus your efforts.
Overall support will include specific feature testing, regression testing, and user testing. Additionally, support tickets may be opened and introduce support costs in ticket management, escalation, and assessment.
Eventually people will start spoofing just to get in, and then you have a much worse problem in your hands. You know the Opera story. There is no excuse to block user agents unless it's commercial software with support contracts on the line.