It really does taste better in those cups and there is actually a reason for it:

They are only given to restaurants who buy actual coke syrup and actual coke machines. If you get soda in a branded cup like that, it generally means that they’re not using some off brand cheapo soda and that their machine keeps the right temps and syrup mix ratios. Usually these places have soda machines with dedicated lines for each type of soda (think the soda machines at fast food places, where each flavor has its own spigot) versus a bar gun system that dispenses all flavors from the same spigot and gives soda a weird “off” taste.

If you stop using coke products, your coke rep will most likely be into your place in very short order to remove any branded products from your store. The people who work for that company are, in my experience, incredibly ruthless when dealing with their territory. Because they’re “Coca Cola so fuck you!”