It's actually a system of load bearing equipment but the word has become synonymous with what is actually called PALS webbing http://en.wikipedia.org/wiki/PALS_webbing
The system allows you to attach ammo and utility pouches on a ladder pattern of webbing. This means that you can have whatever pouches you want in a layout that suits the individual without having to make a "one size fits all" solution.
No we can't that's the point of the tread, both options are available and both have advantages and disadvantages, you look at your requirements and chose your gear to reflect it, for some a Mini Mav for 3 mags is plenty, for others a 1961 rig is required. Just because one is different doesn't make it any less valid.
Molle has the advantage of being modular and can easily and quickly be changed to your personal requirements, the SA vest for example has the advantage of being able to carry a large amount of gear conveniently, comfortably and in the right place when you need it. It really does boil down to your personal choice.
And budget, too- you can get an SAAV or a cheapo Crossdraw-style vest for under £30, but the only viable MOLLE vests are over £50 and over £100 in some cases
Now thats most likely down to the vest not the lack of MOLLE. As is the case with many rigs, even MOLLE, you need one that suits you and if you don't no amount of MOLLE will help that
I have also tended to note that people will usually put the pouches on a MOLLE rig basically in the same place as non-MOLLE rigs but with little change.. for instance I be the location of mag pouches on a low profile chest rig will most likely be the same as they are on my M83 chest rig. Pouches on my MOLLE belt rig are basically in the same places as they would be on PLCE
One point I feel hasn't be pointed out yet is that fact non MOLLE rigs do tend to be lighter and have a lower profile as you don't have the layer of material used the attach items.
However, for plate carriers I would probably suggest MOLLE, especially if they're being issued as you don't need to provide different carriers for different roles