How do you use heroes in campaigns ?

I've recently started to replay the campaigns, and what strikes me this time, but that I never really wondered about before, is that I tend to find heroes a bit useless, and wonder how you are supposed to use them.

The thing is, while heroes have better stats than regular units, it's not enough to make them a one-man army, but in a large army, a hero is barely noticeable despite his good stats. I think that heroes were more noticeable in Age of Empires 1, where population is limited to 50 and armies' size is quite restricted, but in Age of Empires 2 where 125-200 is the norm for population cap, you aren't really going to feel the difference for one particularly strong units. Add to that that many heroes have a "must survive" condition in campaigns, so you aren't going to try too hard with them, and possibly keep them safe far from the action.

In fact, there are only three situations where I find heroes actually useful:

1. In scenarios (or parts of scenario) with very limited number of units, preferably without base building, like the first scenario from Joan of Arc campaign. Thing is, this is the only mission I can think of that meets all the criteria (few units, no base building, and at least one hero).
2. When defending your city. Although, this greatly varies depending on which hero we are talking about, the mission and the difficulty.
3. When you have a small "army" of heroic units (like, when you have a few Archers of the Eyes in the Saladin campaign).

When attacking an enemy, I will take the heroes that can die, but it won't make much of a difference in a large army, and they are quite likely to die unless I micro them to the point I end neglecting regular units, which in my experience isn't the best option.

Oh, and then, you have heroes who are barely better than the regular unit, like Lord de Graville. Not to mention heroes like Jean Bureau, who is actually weaker than the regular unit.

So in the end, I wonder how you are supposed to use heroes in campaigns (or random maps, if you are using a custom script that puts heroes on the map). Really, it's too bad we can't mass them...

TLDR: I find hero units a bit lackluster and am wondering how you are intended to use them in campaigns, because they tend to fall under one or more of these categories:

• The "protagonist" who cannot die (or you lose the game), so you need to keep it safe and can't do much with it besides defending your city, regardless the hero's stats.
• The upgraded unit that looks all nice and shiny, but you won't feel much of the difference within an army.
• The hero that is barely better than a regular unit anyway, if not outright weaker.