what if we weren't winning 4-0, 5-0 and we needed an impact sub like voronin or benayoun. the other three subs are more of a necessity with a cover for a goalkeeper, defender, and a central midfielder. you can't go into a match assuming that you're going to win it 6-0. that's impossible. you go into a match with a plan a, plan b, plan c, plan d etc. etc.
i have always supported the idea of 7 substitute bench with two compulsory academy players. so when the match gets tough, it is not necessary for the gaffer to use these players. however, in situations like this, he can introduce these players for some precious minutes because like the original poster, i believe that playing in the first team improves a players all round game.