Ikbengodniet said:
So? He still used it as a prep race anyway.
Sagan could have got the training effect of 8 days of racing anywhere else, for instance doing the Tour of Britain, as Kwiato did in 2014. Last year Sagan raced 11 days between Tour and WC: Plouay, Quebec, Montreal, European Champs and Eneco. So the fact the he did 8 Vuelta stages in 2015 is irrelevant.
Hushovd completed 17 Vuelta stages in 2010. Gilbert and Evans finished the race the years they won the Worlds.
If all you need is around 10 race days between mid August and WC there's hardly any difference between doing them in the Vuelta or anywhere else. If you need or want more than 15 race days, there is probably no better way to get them than riding the Vuelta; but even in that case with the current race calendar there are alternatives.