Because it's bad behaviour to rely on browser user agents instead of feature detection. It's what led to IE-only sites and is currently leading to Chrome-only sites.
Although, it's not very good in Chrome either :)
Yes, very much so. Especially on mobile, where UA-sniffing is rife.
Here's a very much non-exhaustive sample:
https://bugzilla.mozilla.org/show_bug.cgi?id=1047854
https://bugzilla.mozilla.org/show_bug.cgi?id=975444
https://bugzilla.mozilla.org/show_bug.cgi?id=1322001
https://bugzilla.mozilla.org/show_bug.cgi?id=958575
https://bugzilla.mozilla.org/show_bug.cgi?id=1020740
https://bugzilla.mozilla.org/show_bug.cgi?id=1232656
https://bugzilla.mozilla.org/show_bug.cgi?id=1377499
https://bugzilla.mozilla.org/show_bug.cgi?id=1393109
(funny what percentage of those are google.com sites....)