Again, not saying to change the world to suit the players. Saying don't waste everyone's time with something pointless. If there's a dead-end, say it's a dead-end. But if you're going to start rolling dice and spending fifteen minutes in complex explanations of opening gates and climbing up slimy tunnels, then have a reason for it.
I've DMed similar situations. It's a 'Catch-22'. If I say right away it's a dead end, then I'd be accused of railroading. So to avoid that stigma, I'd have to let them make choices that would appear to be 'wasting time'.
The main problem I see here is the actual splitting of the party.
Once a party splits up, not only does the load double for the DM to provide entertaining choices, but the amount of danger increases for the characters.
The first instinct I'd have is to throw a monster (the forementioned ooze) at the piping-PCs. But, what about the other PCs? They'll need something entertaining. A fight for them is one answer, such that everyone is rolling initiative and participating, but sometimes throwing in a fight feels arbitrary and 'railroady'.
But, having one group in a fight, and another group in another situation (non-fight) creates its own problems:
- Some players feel like a fight is a reward, especially since there is XP involved. The non-piping PCs would feel ripped off unless you presented them with a fight and/or Skill Challenge.
- Often times, however, the non-piping PCs will feel like they should deserve some reward because they are actually progressing plot while the piping-PCs aren't. By 'rewarding' the piping-PCs with a fight or information or treasure, you are establishing a precedent: this game isn't about teamwork.
So let's say both groups get an encounter. Now, if the piping and non-piping PCs are in their own separate fights, there's the danger of overwhelming them with encounters that are too difficult. Since 4th ed is designed for team play (at least in combat), I could easily see a half-PK.
Of course, I probably would have thought to myself 'screw it' and had the pipe open (and rewrite the adventure to have the pipe open at that precise time), then present it as a Skill Challenge to escape unharmed, but some players can see through this decision and think the DM is trying to kill them, instead of making the whole situation interesting.