The main reason you might like a clip on cup holder is because of a lack of space on the table top. If you are having a gathering and people need to squeeze dishes onto the table, having the drinks off it would save you some space. And of course, it will minimize the risk of spills when you have so many people sharing a table with plates everywhere.
Then, these clip on cup holders should also be colorful and visually appealing, unlike the Scrap-Ma-Bob holder, which is more for office use. If you have them in different colors, all the better because then it makes it easier to identify which drink is yours.