How many home grown players a club has isn't that important as very few clubs actually use up their 25 man squad. The only important number is the amount of non-home grown over 21 year olds - you can only have 17 of those.
Not a lot of clubs compete in 4 competitions, not a lot of clubs compete in 4 competitions + extra super cup, world club cup. The clubs that do will have a full squad, and the ones that can win 6 cups a season will have as many top class players as possible.
I think you can bet a team that spends over half a billion in a few years wants to be one of those teams and certainly needs a full squad.
More than half a league has no european football which can easily be 10+ games a season, more than half of the league won't have a significant cup run, which in both cups can be 15 odd games. The amount of times a team hits the Europa league, has a run and see's their league form suffer badly is, well, not surprising. City with a run in the CL to the final, a run in both cups to the final and going to the world club cup could be what, super cup, charity shield, I think hits 31 if the world club cup is 3 games. 31 games compared to a "normal" club having as few as 40 games, 38 league and one in each cup minimum. so 40 vs 69 is the possible difference and that is just club games. How many international games do City players play in vs, Bolton, Sunderland(not as big a difference as you might think at first glance). How many play in summer tournaments, etc, etc.
But again I'll point out the homegrown players give you flexibility.
Lets say City have all 17 slots filled, so they can't buy, Ibra, without leaving someone fairly crucial out of the team. So if and when you can find someone good enough who is also homegrown, if you add them to your squad. IE paying 7mil for Clichy rather than 10mil for Maicon, could be the difference between being able to add someone like Ibra, or being forced to fill out the squad with the best English striker available..... who is no where near as good.
Anyone, any team can find 8 homegrown players, there are thousands of English footballers and hundreds of foreign players like Clichy who count. but there are a handful of TOP homegrown players who don't diminish the quality of the squad by being included, and by using them you leave yourself more room for quality foreign players.
A team going for everything needs the biggest squad possible, and the best quality homegrown players they can lay their hands on, the only really damning thing is that they've bought Milner, Barry, potentially want Walcott. There are better English players, but not available. You look through La Liga and there is enough quality Spanish players throughout the entire table, just more of it at the top. In England outside the top 2(before City were there) the English talent is pathetic, and Utd/Chelsea have their fair share of useless English players anyway.