Get clogged up
(sorry I couldn't help myself)
I was going to say it was put there to keep you occupied, but I like your answer better.
But since someone asked...
Fuel injection works by calculating out how much fuel to squirt into the air so the air-to-fuel ratio in the neighborhood of 14.7:1.* The amount of air that will end up in the combustion chamber during the intake stroke varies with temperature and pressure, so the ECM needs to know those things to get the right answer. (This is why carburetors have to be re-jetted for altitude and changes in the intake and exhaust. FI can compensate for it in real time.)
The five-way tee is used to bring a sample of the pressure in all four of the intake manifolds to the manifold absolute pressure, or MAP, sensor, for measurement. The FI system in the ST may use an average, or Honda may have characterized the whole thing well enough to know when to take a reading for each cylinder. In either case, if the plumbing is partially- or fully-clogged, the the amount of pressure the sensor sees will be different than what it actually is. The ECM, having no way of knowing whether the pressure it's seeing is right or wrong, makes a decision based on what it's given and won't fuel the engine the best way possible. If the clog gets bad enough, the ECM might start seeing measurements that are impossible for the range of altitudes where the bike is designed to operate and will register a fault in the system.
--Mark
* 14.7:1 is a
compromise that nets you the most power for the least fuel consumption. You can make a bit more power by running rich (12.6:1) and consuming more fuel or you can save fuel by running lean (15.4:1).