It's quite simple. Playing a sixth specialist batsman allows you to score more runs.
Of course, there's always the chance a specialist batsmen will get a duck.
But by having more batsmen there's a greater chance that one of them will fire.
It also shortens the tail. It means instead of taking batting into account when picking your bowlers, you can just pick the best 4.
If Alec Stewart was still playing, England could play five bowlers every day of the week. But he's not. So England need to make better use of their current all-rounder. And that means picking Flintoff as a specialist bowler who bats at 7.
Of course there's room to tinker with things. If it's a flat pitch you might need an extra bowler because you'll be in the field longer and batting will probably be easier.
I don't expect to see too many flat pitches in England this upcoming summer.
So pick 6 batsmen, Flintoff, your best gloveman, Monty Panesar and your best two fast bowlers.