Separate steam gauges give a bit of redundancy in the event of a boiler blowout, especially in IMC. Best is a basic pack of steamers paired with a gee-whiz-look-at-this status symbol.
All technological solutions to problems have their pros and cons, both for technical reasons as well as human factors. Because of this (especially the human factors reasons) a number of pilots like to argue the merits of one over the other.
Personally I have about equal time in both and I'll take glass any day of the week.