Many certified glass panels will also have backup steam airspeed and altimeter gauges. Is it necessary? Maybe not if you only fly VFR and can land your plane without looking at airspeed gauge or that annoying backup altimeter.
If you ever experienced an electronic or software "glitch" that rendered your glass panel a big black hole, then you may want a backup steam gauge or two. Having experienced several experimential glass panel glitches over nasty terrain, once during the approaching evening hours, there is no question that I'll keep airspeed and altimeter steam gauges with my nice experimental glass stuff. It's amazing what runs thru a persons mind when that one pretty colorful panel goes blank........ and they do go blank .........