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.

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.

The correct answer is an emphatic "yes". You should infact jump off a cliff as soon as you think your team has lost a fight. The earlier the better, because it denies ult charge from the enemy team and ensures you will be able to regroup as soon as possible. I have seen top 500 players jump off cliffs after only 2 teammates have died in a fight.

Actually, I've seen a top500 player jump off a cliff just because the enemy team used their ults. Why? Because his team wasn't going to use any ults, and he already had his ult fully charged. Him being in the fight would accomplish literally nothing. They just wanted the enemy team to use ults, which they did. After which he regrouped with his team and they used their ults to win the next fight.

Obviously if you can get out of the fight safely without taking any damage, then you should do that. But keep in mind that if you have your ult up, any non-fatal damage you deal to the enemy only helps them by giving them ult charge, so usually it's equally good to jump off a cliff.

For reference, I'm a master league player, but you will quickly see pro players suiciding from lost fights if you watch any stream.

I would say no (with a caveat to be explained in a moment), because a team wipe is a great opportunity for the enemy team; it offers a chance to completely regroup and heal back up, thus charging their supports' Ults, letting them entrench their position, and otherwise making things more difficult for when your team does get there. With most characters, it makes more sense to harass them from a range that you'll be safe at (and take out things like Torbjorn turrets while that's still easy, or punish Bastion for trying to set up in a spot that would be a problem); with many flankers or pickoff characters you might instead take out their supports before they can completely heal the team, to steal the initiative from them on that front and keep some of your team's momentum (alternatively, you might find yourself a good position to hit their back line from when the rest of your team does arrive). If you're a character like Reinhardt, you should pull back to intercept your team (as they're presumably faster than you), or decide on an angle to push from and call out the group up.

Taking yourself takes away options, and I don't think it's as free as you're suggesting - I've certainly gotten elimination credit when a Reinhardt or Winston I've damaged has dropped themselves off the map, which presumably means Ult charge as well (I did not make note of this at the time).

The caveat is this: Some suicide plays, like charging an enemy off the map mid-Ult as Reinhardt, are effective choices when the rest of your team IS around to benefit from what you just did.

Edit: This answer is based on general fighting game knowledge. My knowledge of Overwatch mechanics is slim. That means, the answer is most probably not exactly the best. cough underestimation cough That being said, I'll still keep it in, as people more experienced with Overwatch possibly might still draw something useful out of it. What's good in most fighting games, cannot be totally bad in Overwatch

There is a third possibility: Go for full offense. By doing that, you get more likely to die so you reduce your respawn delay towards the team, and you can flick up some serious damage before you're off. Also, that makes an insanely awesome look in a Let's Play and, if that exists in Overwatch, in the battle replay. (For the awesome look: Remember the LOTR scene where Boromir dies and keeps demolishing orcs upon doing so? Now imagine you can put in an offense strike scene of that quality!)

I have little idea of Overwatch in particular, but that's what I can tell from my general fighting game knowledge.

Also, from what I know about Overwatch so far, I could imagine there is a character with a Nuke Ultimate, about like the Explosion move from Pokemon: The character makes suicide, but deals serious damage to everyone nearby upon doing so. If you're that character, and have your Ultimate ready, then jump in the middle of the enemy group and blow up yourself.