There are several benefits from using those forest patches instead of single trees.
First those forest patches add to realism and gameplay. You can't take a real tank and drive it through a forest with thick trees. The tank needs some speed to waste the trees and this only works if the trees are rather thin and the forest isn't dense. In RL forests are normally blocking tanks. So a whole forest where you just can drive through wouldn't be much realistic.
Normally you would set up an ambush between forests (where the enemy tanks have to move through), but with a forest where the enemy tanks can just drive through you can't do much tactical things.
Another thing is memory consumption. Since one forest patch is one big object it needs less memory than if you fill the same place with single trees. (I'm not talking about the memory of the graphics card or graphics performance but the main RAM or the PC.)
Maybe ppl with less RAM might experience more lag with forest made of single trees than with forest patches because memory needs to be swapped to HD.
Forest patches are also better for planes and choppers since they can oversee a larger area.
Now if the forest is made out of single trees and the maximum number of objects displayed on the screen is 256 (the limit in the OFP config) a much smaller area of forest is displayed.
Also tanks behind some forest might not be displayed because the single trees of the forest in the foreground allready use up the 256 object limit. With forest patches, a much bigger area can be filled with forest without using much objects.
So there ARE reasons why BIS used forest patches beside beeing too lazy to place all the single trees
(In fact, you can be even lazier and use only those rectangular shaped forest patches to place forest and then exchange them with the 2 triangular forest patches with the config. Just look into the original config for the BIS maps for examples.)