I guess the classic blend is some new added to some stronger old in order to bring the lot into condition and possibly for the referment to chew up some of the aldehydes arising from the cask secondary/tertiary. My problem is that any time I have avoided drinking temptation and actually matured a decent old, I have tended to prefer it undiluted!
One area where I can see blending being fairly useful is with sour mashing. With lactic bugs halted by the boil, sourness tends to moderate with maturity (possibly due to ester formation). An appropriate blend at 2 weeks winds up being too unassertive at 3 months.
