Is this a bad thing? For me I don't think so. Yes, it sucks starting in a thirty hour pit, yes it sucks losing all that time that I'll never get back, but it really makes you appreciate what you've been through. Not only that, but as you replay through the game the memory of what has previously been accomplished slowly comes back, and the time spent between story advances becomes less than it was the last time the quest was attempted. Previously convoluted parts of the story line may become clearer, and previously unknown areas and secrets may be found due to a more focused second run of a game. While I don't always want to go back through a game, sometimes a second run can help smooth out the edges of the story, or let's you see more clearly how you end up where you end up.
On the negative side, you are going back through hours upon hours of already accomplished game, so if there was a slow part that didn't really appeal to you the first time, it has to be fought through again. On the 360 this can be a mood killer too, as all those achievements already unlocked won't give you the smug satisfaction of bloop, and makes it seem as if you're playing for nothing, at least until you reach where you last left off. Grinding too can be off putting, as on a second play you generally know what level you need to be around to push on through, and may try to skirt it to more quickly reach further in the game, leading to more deaths and furthering frustrations.
So, in the end, is it really worth it to play back through twenty hours of a game to get to and through the remaining hours? Or is it more worthwhile to forget about what you've learned and press on regardless to finish the game? For me, I'd rather experience a game as a whole, enjoying it from start to finish with no questions besides what is presented to me in the game as opposed to what I should know but have forgotten. I'd rather replay through what I've already seen and done to garner a greater appreciation of where I stopped the last time, to justify the time and effort spent, and also to be more dedicated to finishing the story and wrapping up the tales of these characters whose lives I've gained a vested interest in.