Im not so sure - at this kind of level (where Spain are expected to be world wide) small differences can make all the difference.
Im sure they will sail through their group - even if they don't get a result in the 1st match vs Portugal, but its the 1/4s and Semi's where the managers knowledge of each squad / bench player will come into its own - or if Spain get one or two injuries.
The 1st XI may also each have their own preferences from the bench - which may not always be the best for the collective , even if the right intentions are meant.