I think the G/30 shouldn't be with G/75 .... and in the past were not. But that is a different discussion 
Who cares about blitz?
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Jul 21, 2015
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#2
Jul 21, 2015
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#3
SInce they measure different things, I think they are kind of useful. I'm better at Quick rated games than I am at blitz and if I played enough OTB blitz, my rating would probably be a lot lower than it is.
Of course, the USCF added two new ratings to cover online play, so now there are 6 official ratings, if you include correspondence.
Like any rating, they are only significant in the pool you play in. Ideally, they should help in tournaments (swiss style) to get the stronger players playing each other in the final rounds.