I don't do any real business in the EU, but I'm a fairly succesful online marketer. Being able to flexibly use SaaS businesses is so, so valuable for testing and iterating on marketing plans. I would fight pretty hard against a company policy that limited it, since today's marketing test is tomorrow's major revenue driver.
I think you misunderstand what I was saying. We collect data in our system. We use that data for marketing under legitimate interest. Sometimes marketing would like more analysis done on the data than we have time to implement. They hear about some SaaS business that will take the data and give them a marketing plan (Yay! No work to do!). They ask us to ship over all the data to the SaaS business. Sometimes it's a good idea because the SaaS business is legitimately providing an analysis service. Almost all of the time the SaaS business is providing nothing beneficial and instead just scooping up personal data that they sell. It's difficult for us technical people to explain why we can't just arbitrarily ship data over to some random SaaS. With GDPR it will be much, much, much better. Essentially I think it will shut down the fly by night operations that are just sucking data and offering nothing in return. But on the flip side it will mean that these analysis operations will have to charge a reasonable fee for their services (instead of selling the data they collect). This, in turn, will prompt the marketing people to have to do due diligence because they actually have to spend money out of their budget. No more "It's free, so why not?"
Similarly we sometimes get asked to incorporate silly things into our service because the marketing people think that it will create engagement. Again, these are free SaaS businesses that are scooping up data and selling it. Although I made up the cat emoji thing, it's not that far off what we sometimes get asked to incorporate. With GDPR, those businesses are going to have to charge for their services and that's going to have to come out of our budget. We don't have to argue "We're not shipping our whole customer database over to a SaaS just so we can have cat emojis on the the system". Similarly, it makes our systems simpler because if they really want cat emojis, we can implement them -- it's just not "free" (it never was, but it's hard to have that conversation sometimes).
I probably should have left the SaaS thing out of my explanation because it's confusing and only slightly related to what I was talking about :-). Like I said, we use some great services for marketing and will continue to do so under GDPR.
Do a lot of SaaS businesses sell personal data as their business model? When I think of SaaS I think of paid subscription access to a piece of hosted software.