Discussed this the other week in the pub. Biggest problem is that they never did a back port of TIFKAM to earlier versions of windows (7 at least, Vista maybe?)
Microsoft tried with WPF to make it Vista or higher. Like many other developers I moaned at Microsoft DPEs about this and eventually the decision to do a back port to XP was taken. They have made the same mistake this time and didn't correct themselves.
As a developer my choices are HTML/CSS/JS in a web browser where OS versions don't matter, WPF / Winforms / MFC for a desktop application which will work on everything from XP upwards and finally TIFKAM which will only works on Windows 8. If it's a corporate application it's likely the desktop will be Windows 7 (the new XP)
If Microsoft back ported to Windows 7 a version of TIFKAM then I'd think about developing for it but until then I'm ignoring it as a waste of my time.
After buying a new laptop for home use (Lenovo Yoga) recently when it's in tablet mode the TIFKAM does actually make a reasonable experience (and the existing windows desktop was never going to provide that), the shame is when I'm in "laptop" mode it feels "clunky" as I find the switch from desktop to menu jarring. I'm going to keep trying until after Christmas particularly as this time I have a touchscreen but I suspect I'll probably give in and install a start menu replacement.