It is bad enough that third party aggregators get the MLS feeds and then sell the leads to the listings that you worked to procure back to you, the agent. 
1. But did you know that a homeowner can list their property as a FSBO on Zillow too? Yes, for FREE, they can list their house for sale by owner on their website, in case you did not know that.
2. If you have a widget from them ( see, I don't want to use their name over and over again- because that helps their branding on Google) on your website you are telling Google that they are the authority for homes for sale. Of course, you are not as big as they are, so the authority benefit goes to that link to them and not back to you.
3. They bought Postlets. Did you know that whenever you create a postlets flier and you have that on the internet, there is a link back to their website at the very bottom of the postlet under the flier?
From an SEO standpoint – these are not in your best interest.
From a listing agent standpoint- this is not in your best interest.
I know, many agents tell me they have to play the game or they lose business. I have never believed that one must compromise their principles, values or business in order to make a profit. There is always another way.
Of course, it is all up to you how you play the game and how you run your business. But real estate agents who don't pay attention to the actions they take online are the ones who begin to wonder why they no longer have their website ranking on the first page of Google, and guess who has taken your place! Agents are the reason for their own loss of control of their inventory.