When I first bought a set of parallels (the 20 pair set) I thought I probably wouldn't feel the need for further sets. But I did, more than once, and the needing is probably not yet over.
Yes, you can get by with a limited selection [they can, after all, be stacked one on top of another if necessary, and you can always make supplements yourself], but the more parallels you have, the quicker and more convenient it becomes to achieve all of those random and unique set-ups life throws in your way.
What would be a useful supplement to the Arc range is a set of very thin ones, viz around 1/16" thick. When you're drilling right through a small part close to its edge I find it's preferable not to have to remove the parallels before drilling, especially if you're drilling multiple parts the same. Of course, the swarf gutter in most vices won't allow you to put your thin parallel right at the bottom against the fixed jaw, but once you're accustomed to stacking that's easily solved.