I somewhat agree with your point. But there are always 2 locos available for Gatimaan (out of #30014, #30007 and #30086) at NZM ETS. Rest all passenger locos are 130 kmph fit, hence no issues of loco availability for Rajdhani and Shatabdi. So I don't think this particular loco is "being" wasted with Poorbiya, although I'd agree that it is not "optimal" utilisation of resources.