Barry you are right about the time valid comment - I have removed it from my previous post.
Back to the discussion, which incidently is about pilots identing on first call and not ident methods.
Sometimes pilots ident by accident in normal flight (the might put something down on the button or put their hand on it by accident)
If you work on a sector where you regularly have aircraft joining controlled airspace from an aerodrome that has no SSR (and often no radar at all) it is imperitive that a/c are indentified.
If a pilot checks in already identing and some other aircraft in the vicinity is also identing, then it has to be requested again.
If we stick to SOP, even if there are no other a/c, and make sure pilots do too, then it reduces the opportunity of a 'cock up' that we don't notice during a busy period.