I believe storylines are important. A lot. But not to the extent that rest of the stuff remains underdeveloped. For example, I thought KotOR 2's graphics and gameplay needed more, while the story had gone way ahead it's time. KotOR 1 is a good example of how a good story works with good gameplay.
Secondly, some games should not, in any case, have a story. Yes, there are plenty of good games, that haven't had a storyline at all. Unless you count the historical campaigns as a storyline, I think AoE 2 was exccellent, as are the Civilization games and the Unreal Tournament games, which purposefully don't have a storyline.
AoE 3 introduced a storyline, and it failed miserably. With good reason, too.
Final Verdict: Games that are taking storylines seriously, should work on it to the fullest. Games that can do without storylines, must omit them completely. That is the secret of a good game.