firstmark,
Let's not confuse "exclusivity" with allowing ads to be used as a default for another network.
You are right that exclusive deals are not for most sites, especially small to medium sized, and with a general, broad based audience. For most sites, entering an exclusive is a bad deal because only a small fraction of the inventory may be sold, and they are prohibited from selling it through other sources.
However, whether or not a network allows its ads to be used as a default is another subject. It may be true that a lot of networks do allow this... about half of networks I am aware of... more than half if it can be accomplished without modifying the HTML code in any way. If a network has a policy of not allowing this, that doesn't mean you can't run other advertising on the page (as with an exclusive), simply that that particular network does not want the default traffic from another network. Often, networks with this policy actually have HIGHER CPM ratios and average CPM net to publishers, because they deliver higher quality traffic to the advertisers, keeping the higher paying ads on your site.
One pitfall you forgot to mention, with the networks that allow otheir code to be used as a default.... eventually, if they notice that they are getting low quality (that "undesirable" traffic you referred to), the publisher will see fewer and fewer of the higher paying ads over a period of time. The network may not care that the traffic comes from a default, but it does care about the quality. Generally, networks that enforce quality have better paying ads overall.
Publishers can still have the benefit of of using several ad sources, as I suggested in my last post, through a combination of using some networks as defaults (when permitted) and dividing up inventory on the front end between multiple sources, in an effort to increase their unique/repeat ratio to each source.