I think the stockpiling is certainly part of planning your War and long term strategy. Without the resources you can' t maximize your IC and other expenses. Having a stockpile of Supplies allows you the flexibility to Maximize your IC while letting the Stockpiles dwindle. So if it is an emergency you can divert the resources to say production or reinforcements, or province repair at the expense of Supply.
Having the a lot of one resource should allow you to trade for other resources, which many nations need to do to survive or grow their economy. Minor nations and puppets have difficulty managing their resources. If one country only has say Energy, lots of it, and nothing else, at least they have a chance with a large surplus and stockpile of Energy to trade for everything else like Supplies or Oil. If you cap this out on these countries it could severely affect the rate at which they could grow. Making Minors and Puppets even less effective then they already are, especially under AI control.
If anything is done to resources and limits to how many you can have, I would say it should be a small percentage decrease over time. In theory, we can Stockpile, OIL, METAL, FUEL, and RARE MATERIALS in Real Life and countries do this all the time. If this stockpile Surplus remains then it would be realistic to have some decay on them. This is something I would support but do not support capping them or changing the limit from where it is now.