Adding vegetables near the end of cooking gives you a better product.
I will pose this question: When making a vegetable "stock," how long do you cook it for? No more than 1-2 hours, right? Why? Because if you cook it longer, the vegetables turn to total much, the flavor becomes muddy and muted, and it loses almost all of its vibrancy. If you don't believe me, then try it at home. Make a veg stock and simmer it for 1 hour, and make one and simmer it for 4 or 5. You'll see what I mean.
The same logic applies to adding vegetables to any stock. If you go ahead and simmer mirepoix for 8 hours while making beef stock, what is the point? Most of the vegetal aroma and flavor will be lost by the time the stock is done.
Now. plenty of chefs make stock with mirepoix added from the start, and the product turns out good. There is nothing "wrong" with doing it like that, per se, but you wouldn't get the best possible product doing it like that.
It is another level of technique you can add to the cooking to continue to elevate the stock. I would rather have a properly made stock with the mirepoix added in the beginning than an improperly made one with the veg. added at the end.
IMO, stock should be neutral. Stock is meant to be a backbone to support other flavors, not to be a flavor in and of itself. If you are looking for something with, say, a chicken-ey flavor, then you want chicken BROTH, not stock. Want to make chicken noodle soup? Use chicken broth. Or, fortify some chicken stock with chicken meat and fresh veg.
I don't want my red wine sauce to taste like chicken, do I?
Stock is made from bones, broth is made from meat. A lot of people, especially at home, make a stock/broth hybrid (that is, a cut up chicken containing meat and bones) and call it stock. It is absolutely fine, but doesn't, at least IMO, make a true stock. Again, doesn't mean it isn't GOOD, just not truly a stock.
The blanching, skimming, re-wetting, etc. can all be discussed in another thread, and at length, lol.