So the terrorists were working on a "towering inferno" scenario only. In that case, to maximize the effects of the infernos, they would want someone to sabotage the automatic sprinkler systems. They would not need or want to carry in more demo to wreck the building: just turn some valves, or even change some signs to indicate "valve open" when it's closed etc. Or chain or lock shut some valves in dusty rarely visited maintenance rooms. A couple of "kryptonite locks" for example could delay the firefighters for a long time.
Plus, the aircraft probably broke the pipes coming in anyway, which would do a number on water pressure, and the massive fire would overwhelm whatever water the broken system could put out.
Some old buildings used to pump water up into a storage tank located at the top of the building, and then the water would flow down, giving you all the pressure you need if the tank was kept topped off. In that case a break in the lines would send a lot of water gushing down at the break. Once the tank was drained though, you were in trouble if you couldn't pump more water up. Or, if someone rigged it so that you though the tank was full when it was not, there would be no emergency water reservoir. I don't know if the WTC had such a system; I guess not since technology has improved things since then.
I wasn't thinking of something tinfoil- I wasn't at all surprised to see the buildings come down- I thought the buildings would collapse from the stress, shifting towards the weakened areas like a coke can collapsing under your foot when tapped. Once it was obvious how hot it was and how long it was going to keep burning, it was a given the steel would fail. I'm just wondering if there could have been some other reason to have people on-site than just fiddling with water sprinklers- unless the terrorists were just doing it as an insurance policy, or to have souvenier photos made for their sick little videos so they could gloat.