Because then the advertising agencies will have a way to make content people want to read and share and therefore they can rely on people sharing it, at no cost to them.
However, ad agencies typically get paid more for spending more, so I don't think a "no cost" campaign works well for covering their costs.
Of course there's probably agencies out there that do/will pitch this type of campaign, and charge for the ideation + production. But virality is usually an empty promise.
Alternatively, increasing the k-factor on an already successful campaign is something agencies will want, so I believe they will fork out for these sponsored placements regardless.
All told, I really do think some secret sauce for targeting (aided by Twitter's/FB's data insights) plus some cool content produced by the brands/agencies will win out as the monetization strategy for social networks.
Why?
Because then the advertising agencies will have a way to make content people want to read and share and therefore they can rely on people sharing it, at no cost to them.