What is the point of using 3-point coins in a base-ten coin game? There are 5s and 10s, and you can almost never exchange the two for any multiples of 3.
I agree with Alex's answer about hiding score. It makes it more difficult to tell at a glance how many VP someone has.
But it's helpful even without that, just to use fewer one-point coins. Remember, the varying denominations aren't just to make it easy to exchange coins. You don't have to do that, and if you're trying to hide your score, you probably shouldn't. The different values also just help avoid needing a bunch of one-point coins for scores that are 3 or 4 mod 5. That makes it a little easier to hand out the coins, and also means that you're less likely to run out of one-point coins and have to ask players to trade in coins.