It's not as big a deal on Windows where the look+feel of various applications was always inconsistent as hell -- remember when Trillian was the most popular IM client, it seemed like in those days virtually every application handled windowing differently, it was terrible.

But it's a bigger issue on OS X and in Linux. Since both have functionality to cycle through windows of a given application, tabs become less important. Of course, we still end up with visual clutter, so even luddites like myself use tabs now and again. But this should really be implemented as an OS feature, not on a per-application basis.

Google, of course, vehemently disagrees, since they implement their own terrible look-and-feel everywhere. (see: crappy windowing inside of GMail, crappy windowing behavior of Hangouts, etc).