Cars are kind of like Dive computers... Air only computers are like manual transmissions... they're for the guys who can't afford or don't want to pay for Nitrox.
You know the first question of the uninformed is about the price of the class. When an instructor doesn't teach the tables, the price of the class seems to decrease. That instructor figures they are spending less time teaching, so everyone benefits.
Just one of the downsides is the fact that some of the people in the class are too cheap to rent or buy the proper gear when they get out of the class. They'll get gauges and they won't have a clue how to use them properly. Spending money on computers is a wasted expense in their mind. The instructor/agency should be ashamed of the problem they just created.
If you want, you can call all of your local LDS's and see if they use gauges on their rental equipment, or is there an additional charge for computers. If you strictly dive in the Carribean, that's where you should check.
The reasons for teaching the tables are soundly endorsed. Teaching people how to use a computer is fine, but the student needs to realize that the tables are for a lot more important than just for historical purposes.
The student quickly realizes that there are many different computers out there and they don't all work the same way. Renting a computer every time will frustrate a diver, when that person can't figure out the different nuances. So, they quickly learn that purchasing their own computer is their only option. Now that cheap in class just became a moot point, because they can't dive safely without that crutch. albeit a useful crutch. If you are trained properly, you can still dive safely without a computer.