Never ever leave your suspension unloaded!
A 2-post is a very good lift to work on your car, but its totally unsuitable for storing a car (1 or 2 weeks wont hurt, but months on end is a nono).
Within your suspension, there are loads of bushings (almost all of them on a stock car) that are fitted with pretension, or better said, no tension when the car sits at ride height.
When the car doesnt sit at ride height, those rubber bushings sit twisted under tension in their casing/control arm/shockabsorber. Over time they will tear.
Thats why in all workshop manuals is shown whether a bushing should be fitten with pretension (or snugged up with the wheels on the ground) or not.
Even with lowering a car, one should retension those bushings to the adjusted heights.
So my guess is that the 50% that say that suspension shouldnt stay extended for prolongued time are probably the 50% that have actually read a workshop manual in their life and have actual hands on experience working on cars 
So imho its very simple, if you buy a lift to store cars, get a 4 post. If you buy a lift to work on cars, get a 2 post
(although 4 posts are better to do very small jobs (a car is more easily put on a 4 post lift) or bodywork, so each has its place)
If you have the room....get both