Plenum cable is expensive, can't I just install the cheap stuff?
Not all installations require plenum cable. But if the fire code requires plenum cable, the answer is NO. You must use plenum cable.
For some customers, being frugal is an act of personal discipline that can overcome even their basic fears of fire and agonizing death from inhaling poisonous smoke. They compare the prices of plenum cable with the prices of riser cable (or even residential general use cable), assume that their office buildings won't catch on fire, and demand that the "cheap stuff" be installed.
Any reputable installer knows that this is a terrible idea. The hard part is getting the customer to understand this.
The best policy is to be blunt and point to the pocketbook. If the cable is not up to code, the fire marshall or building inspector will require all of it to be removed. It doesn't matter if the entire staff was supposed to move into the new building tomorrow, it doesn't matter if the walls are already in place, and it doesn't matter if someone just spent $10,000 (or more) on the new network; all of the "cheap stuff" will have to go. Then one gets to pay for the cable that should have been used in the first place in addition to the "cheap stuff" that had to be ripped out and thrown away.
It is always more frugal to follow the building codes than to ignore them because if you don't then you may end up wasting lots of time and money by having an installation done twice.