I see a lot of people having ult but not ulting. This is usually a mistake.

The purpose of an ult is to win you rounds by tipping things into your favor. Given that, why wouldn't you want to have that advantage as often as you can? Even better if the rest of your team can do so as well.

~~Once you use~~ After your ult, every kill, every plant, and every ult orb you get will go towards your next ult. If you save your ult, all of those potential ult points are wasted.

If you can turn an 80% chance of winning a round into a near 100% by ulting, just go for it and secure the win. There's no point in complicating it and risking a loss.

Rez to turn 3v2s into 4v2s, run it back for a free extra life and info, empress to gain a massive boost in dps and heal and reposition for free, lockdown to win automatically.

Your ult is there for you to benefit from it. If you use your ult now, you will get more ults off throughout the course of the whole match than if you save it.

Use it or lose it. 'It' being all possible future ults

Win the round (you'll probably get kills in the process too), pick up the orbs and do it again