A company could decide to not sell to the largest retailer, I don't think anyone is denying that. But all that does is move the potential monopolizer one tier down. By definition the monopolizer is taking away the markets power to set pricing as well. In the MAP model, the chance of there being a monopolizer is reduced, therefor both the retailers operate on a more even field (each retailer will be selling from multiple manufactures, and potentially multiple product categories) and then the brand of the manufacturer has to compete on that level playing field with other manufactures in their category.
I'm seeing it like 3D chess, the retailers compete, and the manufactures compete, and each does use the other to aid it when possible. That gives the end consumer the best chance of having the best marketplace overall. Yes, it will mean that overall prices will be slightly higher, but that means that a single retailer cannot suck a brand dry and kill them off for their own gain.
If you presume all are operating in good faith, then MAP is not required, but if a retailer decides to monopolize a brand for its own profit, they damage that brand, even if not fatally. If the bar for entry is high enough, then that weeds out those who are not planning on being viable long term, but as 42Blades stated, the low bar for entry right now means that little sellers can pop up like weeds and damage the legacy retailers, and in doing so damage the reputation of the brands. Even if one seller doesn't form that monopoly on their own, the market might, and market forces have killed brands before. More sales doesn't always mean long term success, especially in a reputation based marketplace. Also the low bar marketplace also makes clones easier to sell and more profitable, therefor damaging the brands and harming the manufacturers.
As an aside, Daniel I think you have made your points well, just that your perspective doesn't seem to take into account the effect of online sales. It could also be that there are certain definitions that are not shared, or that your market experience doesn't translate 1:1 with knife retailing, I don't know. I'm certainly not an expert, just trying to frame a debate as best I can hope to understand it better myself, as well as add to the general knowledge of the community. Someone1 was out of line, which is unfortunate because he seemed to be making good strides away from the start he made. Oh well.
Lastly, I realize that there might be an ideological component to the debate as it stands. I hope I don't have an ideological bias, I'm trying to come at this from as realist a perspective as I can with the knowledge I have, but that might also be part of why these threads go this way. Not to say there in anything inherently wrong with an ideological discussion, but both parties need to be doing that, otherwise we are speaking different languages.