I say there is a pretty noticeable difference in the outcome.
I add the veg in the last hour of cooking the stock. The aroma of the vegetables is at the peak about 1 hour into cooking, after that the aroma fades and gets a mushy, overcooked vegetable taste. You said it yourself--why don't we cook vegetable stock for 5 hours? Cause it would taste nasty. Why would you want that nasty vegetable taste in your chicken stock or veal stock? 1 hour is plenty of time for the flavor and aroma to come out from the vegetables. Just like veg. stock.
If you don't believe me try cooking a veg stock for 4 hours and tell me what you think.
On the other hand, you can just omit the veg. from the stock and keep it basic and neutral. Veg flavorings, as well as herbs and spices, can always be added later to enhance flavors of sauces and soups. May be beneficial to just have a "blank" stock with which to work with.
The only "pro" I can think of for adding veg in the beginning is that it's a little easier not having to think about it.
But I would definately add the veg 1 to 2 hours (max) before I pull the stock off.