Is there ever a time where I should just suicide?

Why are the only two options to kill yourself or let yourself get killed in an outmatched situation? Simply retreat if the situation is no longer in your favour and wait for your team to respawn. If it means losing the first point, then so be it.

Of course, if the enemy is attacking the last point, throw yourself on the capture area even if you die a second after.

As a rule of thumb, no.

The situation may differ depending on whether you're attacking or defending, but the time it takes to die, respawn and run back is time you could be spending diverting enemy attention, rounding up health packs (to zone/deny your enemy getting them when the next team fight starts), or scouting for teleporters/turrets/flankers.

One thing I will mention though, is that in a situation where it's suicide or die to the enemy without the option of retreating/regrouping (let's say you overextended and can't backtrack without getting pipped by a turret or sniper, and you're watching a Reinhardt trudging towards you); suiciding is the better of the two options.

This is because if you let that Reinhardt hammer your face in, he gets ultimate charge, which stacks the odds against your next engagement. If you suicide, the respawn timer is the same, and he is ~10% further from his ultimate because you decided the cliff would treat you better than his beatstick.

In "Mystery Heroes" mode (one of the weekly brawls, where your hero is randomly chosen each life), it can be useful to suicide to get a new hero. I've done this when, for instance, I was one of four(!) Symmettras on the team. If done before the round starts, it's basically a free mulligan.

In a normal game, the only time suiciding would make sense would be if retreating wasn't an option. The maps are well-designed enough that this should never happen.