Episode 5 uses a point system on the winning path, similar to the system used in WC1. There is a number of points to be won at various times, and you need a given number of points to get the winning ending (the only alternative, though, is the middle ending; the *really* losing ending is unobtainable on the winning path - sorry, the Firekka is too small a fish to make that kind of difference in this battle). The first opportunities to score points come long before Episode 5 - you get points for saving the Verdun in Episode 3, and destroying the Tarvakh in Episode 4). Destroying the Tarvakh, in addition to giving you points, also allows you to score more during Episode 5 itself, because otherwise, that Snakeir won't be there at all (it'll be busy escorting the damaged Tarvakh back into Kilrathi territory).
Anyway - if you've destroyed the Tarvakh, you can afford a few failures. If you've only damaged the Tarvakh... well, you're on the edge, then. IIRC, if you go into Episode 5 with the Verdun lost and the Tarvakh only damaged, you'll either need to get 100% success in all other scoring opportunities (which doesn't necessarily mean *all* objectives - for example, Targus usually don't contribute to your points, even when they're an objective), or it may even be that the winning ending is completely out of reach.
The winning ending *is* more upbeat than Fleet Action - not by much, but it is. That's why, if the Tarvakh gets away, as it did in Fleet Action, it takes a really superhuman effort to deal more damage to the Kilrathi than was done in the book. It's a bit counterintuitive, but it makes sense, all things considered. You'd think that merely driving away a supercarrier is good enough when you're fighting to save Earth... but the winning ending also implies that Confed will be able to counterattack, and a damaged Tarvakh along with that Snakeir could very well prevent that.