While you're right - in this case it might be perfectly acceptable to decide "I'll use unreliable user-agent sniffing to handle 80%/90%/99%/99.999% of my traffic, and accept some small cases where a slower option occasionally needs to be used, or possibly accept minor font differences on odd-ball browsers".

Old wives tale.

Not to mention there are more ways to tell apart browsers than user agent sniffing.