No, you don't need unlimited developer time to do feature detection. It's just good practice, after it's ingrained it doesn't take any more time than not doing it.
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.