I think it depends on the artist and the compilation, you just can't make a sweeping judgement like that. For example...

Clapton's Crossroads 4 CD set. Didn't even begin to scratch the surface of an outstanding career. A good place to start for a Clapton rookie.

Jimmy Buffets : Boats, Beaches, Bars, Balads: A great retrospective of an artist work. Jimmy writes "One shot" songs, he doesn't do "Concept albums", consequently this format works for him.

Echoes ; Pink Floyd....Not even close, As we all know PF albums are complete works in and of themselves.

Electric Light Orchestra : Flashback... again like Buffet this works for a band that produced a string of hits. No loss of concept, why? No concept ot loose.. great set.

Zeppelin boxed set, (4CD).. maybe if it was 8 CD's they might capture the grandure that is (was) Zeppelin, Couldn't do the band justice.

Beatles Anthology I, II, III (6 CD's total), as a set, not quite there, but combine with the 2 CD set of "Beatles Live at the BBC" and you have a great histoty of a history making band.

I would not be as quick to condem the sets as you my friend....

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