Novel idea by OP..but the point is when your five best batsmen cannot prevent the team from losing how it would matter if your next not so good 5 would help in preventing the loss...i think it will not work..bad team would still end up losing 3-0..
There are reasons why it could work. The Batting order will become important, but just because selectors pick the 5 or 6 regular batsman in the team, it doesn't mean they are actually the 5 or 6 best batsman.
If 9 or so are picked it enables:
a) the batsman who are in the top 5 or 6 who are under pressure to really knuckle down knowing that if they fail they aren't going to be in the top 6 when the team reverts to traditional form.
b) it gives the bats at 7 to 9 or 10 an opportunity to perform well and be in the best regular XI
c) it gives the other bats experience (while losing out on bowling experience which is a negative).
d) it IMO will improve some batters techniques as they realize that hitting the ball along the ground actually helps, you see a fair few soft shots these days, hopefully this mindset would lessen the soft dismissals.
Also, not a brilliant reason to do this move, but it is likely that fielding standards will improve which saves SOME runs.
Honestly though, if the team with minimal bowlers bowl negatively, have a third man, a ring field etc, the batting team will need to bat well to score runs and it could at times create some soft dismissals which a Herath or Ashwin (if India did this) could take advantage of.
BTW, just out of interest, if you think 300 or 350 is a safe lead to declare ahead with the fourth innings to come, how much would that increase if you played 9 or 10 bats?