Maybe. But ET customers are not small and already have CRM of some sort, and there are a number of 3rd party integrations (I know the guy who did the SLX one). So current ET customers would have to change CRM -- I doubt this would be a stampede.
ET already has integrations to sfdc, so I don't see a huge uptake among sfdc customers unless those customers change their marketing automation needs. Again, I don't see the stampede.
sfdc used $1 bn in borrowed money for the purchase. It claims to want to grow its marketing revenues 10x to $1 bn. Now, they do have several other MA pieces already, so maybe the synergy of the pieces working together will generate this huge growth. And MA has its own monthly service stream tied to marketing activity, so maybe the growth is there.
It just strikes me like a lot of $$$ from a company who is basically B/E, using borrowed $$ to pay huge multiple (8x revenue) on a market that is growing well but not exploding. Sounds like a bubble to me.