There are strong formations out there that only need the attacking players though, such as 4-5-1 attacking and 5-2-3 attacking.
5-2-3 attacking was my formation of choice and it was quite hard to beat. I just felt it lack a slight edge when competing with non-attacking formations over the course of the season. Do recall Fools United winning a major with it, although it doesn't appear too often in completion winning sides.
Instant subbing removes chance of players out of position, but Ag still makes a good point about having your bases covered. If possible I'd like to have an anyside DEF, MID & STC on the bench, all changed off early so they have full gelling.
Apart from your GK, any injury in the game will be covered and gelling unaffected for your next match, should there not be more than one injury to the same position. In certain scenarios this wouldnt be possible in an attacking formation.