The context makes it pretty clear that Eggo and Manwich in the commercials don't refer generically to waffles and sloppy joes. The danger is that the public may begin to misuse the trademarks unless those usages are policed. Few mark holders police their mark more aggressively than does Coca-Cola.
Interesting point re "context", but I don't think "context" is really the operative term here if that means the context in the ad. The context of "Leggo my eggo" is someone having a tug of war with the Eggo brand waffle, as I recall it, a context which definitely shows a nominative use of eggo to refer to the specific item being pulled. And, if "context" is understood to mean the mere the fact that the term is used in an ad, that "context" won't protect any term from getting picked up for generic use. Instead, I think the operative force here is the fact that there just isn't any great need for a new generic term for these products, and that's what keeps the use from being too risky.
I could run an ad campaign saying "Buy Squirrel Springers, the new craze!" for my newly invented spring shoes for squirrels, but the fact that people can tell it's an ad won't keep them from using "springers" to describe any little spring shoes for the beasties if they don't already have a better generic term available. I'd do much better to say "Buy Squirrel Springer brand hopping shoes!"
But Isaac is quite correct that policing is important. It is the trademark owner's job monitor usage of the mark not only to prevent infringement, but also to discourage the public from using a trademark as a shorthand generic term for the product sold under the mark. (Think of the Xerox brand copier and the company's ad campaign: "You can't Xerox a Xerox on a Xerox.”)
So if I see columnists talking about the new craze of using squirrel springers and how many brands of these are coming out, I should inform them that those others are just hopping shoes for squirrels and not "Squirrel Springers"
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Sorry I wasn't clear re the Eggo/Manwhich example. I didn't mean to say that Eggo, e.g., would have become a generic term for waffle. I was groping for a way to explain why that those ads, which violate the "don't use a trademark as a noun rule", could be a somewhat risky usage, but still not result in the public picking up on the use and making it into a generic term for that type of waffle.
Eggos, as I understand them, are always frozen, small and round. So if small, round, frozen waffles were a really hot item the public clamored for, then "Eggo" theoretically might have fit the bill and become a generic term for that subset of waffle -- but not for waffles generally. That is, if there were a public need for a word which meant "small round waffle", "eggo" would have been at risk to become that term. But, as it happens, there wasn't such a need to distinguish small round waffles from the others and so the regular word "waffle" worked perfectly well and anyone wanting to describe a small round one would just use those descriptors with the broader term to cover the subset.
Same thing with "Manwich". For the term to become generic there would have had to have been a need for a word to describe the type of sandwich a "Manwhich" is. This would only work if 1) Manwich means as particular type/size of sandwich (which it is, as I gather from Isaac's reply, although I'm really much more familiar with waffles.

) and if 2) the public has a need for a word to describe that type of sandwich. Otherwise, the broader generic term continues in use and the trademark term doesn't get used as a generic word for the subset. If sloppy joe is indeed what a Manwhich is, then there is already a term in use for that subset.
Interesting thought, though...I wonder if Sloppy Joe was originally someone's trademark, before becoming a generic term.