Check PH pg 130 passing through, designated exceptions. Some creaturres break the above rules. For example a gelatinous cube, fills the area it occupies to a height of 10'. A creature can't move through an area occupied by a cube, even with the tumble skill or similar special ability.
Basically since that is an example, maybe the most obvious one, but an example of where you can't tumble through these situations can occur. And that is where the DM comes in. For example when I use wizards as the big bads, at lower levels I give them an honor guard of dorks who all carry tower shields. They then block the corridor with tower shields, and unless you can high jump 6' and still clear the goons distance wise, tumble just isn't an option. Basically the party has to resort to missile fire, hah I got protection from arrows up, spells this could hurt, or fight there way through the bad guys the old fashioned way, which isn't easy when they got tower shields, even though they usually are suckers. Meanwhile Mr. Evil in the background a higher than party level wiz/sor giggles insanley as he pummles the party with spells. And guess what tumble then means nothing in this fight.
Tumble isn't some secret mystery so enemies preparing for it isn't far fetched. And if you prepare for it, tumble really isn't that potent. Tumble only really effects things in my games, either where I don't care enough to get the enemies to prepare(variious goon and mook groups whose tactics aren't much the issue), a surprise encounter, or when the players come up with something really clever.