Some people love the Bioware games, but I find it hard to be immersed in a branching storyline.
That's because I know that under the hood the game is still on rails. The difference is that every now and then there will be a switch in the track where one must choose, and the choice is irreversible. I'll be travelling along nicely towards Chicago, and then a switch comes along where if I make the wrong choice I'll end up in Cleveland—or even worse,
Akron. Then I'll have to restore from an old save point, because once you've chosen the wrong branch (as they say in Maine), "You can't get there from here."
Then there's the issue of replay value—games that are on rails have limited replay value. Branching simply hides that a bit, but it doesn't fundamentally increase the replay value.
There's also a cost issue which effects the amount of content in the game: each branch adds to the cost to produce the game, so naturally companies seek to minimize the amount of branching in the plot.
Branching storylines are familiar to the Hollywood types who have infiltrated game companies, and many of these people think of games as movies that you can play, with all the inflexibility that's implied.
As an alternative, I'd like to see more effort spent on dynamic, mutable storylines that change in response to the player. To fully realize this vision would require an AI breakthrough that's a long way off, but I think a lot could be done with today's technology. A move towards algorithmically-controlled plots would buck the trend of static stories that's preoccupied the industry for many years now.
In some ways Bioware's upcoming MMO, Star Wars: The Old Republic, represents the culmination of the on-rails branching storyline genre because every NPC is fully voiced. Not only will it be on rails, but the track will be cast in concrete since changing any dialog will require a return to the recording studio instead of just a text edit. This portends the most rigid and unchanging theme park MMO to date.
This is a big reason why I have no interest in Star Wars: The Old Republic—it's just another theme park.