If you know better way to obtain browser capabilities than checking its UA, please tell me because i don't.
Different browsers behave differently, this is reality and you can't do anything with it. As webdev you must build your code to handle with it. And you need to sniff UA to know how to deal with given browser.
In what order will it generate all these mousedown, mouseup, mousemove, mouseenter, mouseleave while user does drag-n-drop?
Will it generate left-button-up event before right-button-down when user does long tap in mobile browser?
Does it support CSS3 with border-radius, gradients and CSS transitions? (server should generate completely different html depending on this)
Does it use W3C box model or MS's one?
Does it support dropping file onto html page?
Needless to say about all these 1px differences while handling floats, negative margins, etc.
You can build some sites universally without this knowledge. But not really complex things.
So, UA sniffing is okay. It is sad, of course, that servers have to resort to such ugly way, but this is reality and you can't blame webdevs. This is not lack of competence. On the contrary, this is indication of their competence and amount of human-hours spent to adopt their site to work best on most popular browsers.

Another question is how to deal with unsupported browser (or browser google just havn't tested their site with). I agree that there should be some link saying "ok, it will be broken but let me in anyway" like it was in Google Docs.
But this is matter of marketing and politics. Many ordinary users will think better about Google if G+ will say "Sorry, your device is unsupported, plz use G+ on your desktop" than if it will let them in but give them completely broken experience. Because in this case they will say "G+ site is broken and unreliable and G+ developers are not smart enough to fix these obvious bugs".
This is not about geeks, this is about regular users. You know, many, many of them will never understand these strange statements like "THIS SOFTWARE IS PROVIDED "AS IS" AND WE DON'T GIVE ANY WARRANTY". But they will kindly understand "Sorry, your rare and unusual device is unsupported" just like they understand that their tiny mp3 player doesn't support CD discs and magnetic tapes.

Sorry for offtopic