A lot of people I play with like hold their ults because they "don't think they're worth it," and I get it, you just got it and don't want to waste it. But by not using it, you are also wasting it. I play a lot of Brimstone, and enemy team would say "you wasted that ult just to kill me" but by killing them I won the round, and in some games I have as many as 4 ults in a half if i'm really playing well.

It is much better to have ult frequently than to only have 1 ult usage per half.

Edit: Everything requires some thought too, if you're sage and you got res you don't need to ult just because, but you might be able to coordinate with your team for a more aggressive push as you have res.