They're not a waste of money, but they're only needed in specific situations. A healthy guinea pig that's getting good pellets, hay and veggies doesn't need one. A senior pig, or one recovering from surgery or illness, or one that has been malnourished may benefit from small doses of a multi-vitamin.
But, and this is important, there are fat soluble vitamins in multi-vitamin preparations, and guinea pigs are just little balls of fat and fur. It's very possible for them to wind up with toxic levels of those fat soluble vitamins.
I've given them twice to two of my pigs, once each. I gave only half the listed dosage, and only 3-4 times per week. And as soon as the pig was back to health, I stopped them.
The only ones I know of are the Oasis Vita Drops, and they're what I used.