Not only is it a valid concern, but it's a business model.
This is one of the reasons why people like me are extremely reluctant to use these (often) free web services.. the prospect of them closing up shop and leaving you and your data in limbo seems quite high -- and it seems like it's often part of the business model.
Basically a company creates a web service and get as many people using it as possible by keeping it free. [as a side note, make sure to call it "In Beta" forever so no one can complain about anything] They don't worry about profits yet. Then at some point they say we have all these users, now is there a way to make a profit on it -- via advertising or whatever. If not, then just close up shop and move on. And the user is out of luck.
Offering your product/service for free is not a business model. If there is no sale, there is no business.
Giving something away to establish credibility, or to identify a new market, is one thing. But I think that is also the major flaw in most web-based "business" plans. Companies offering these services seem to forget that the willingness of someone to pay for something is one of the best reality checks available for how well your product or service meets a genuine need. If people are willing to "do without" if they have to pay you for something, then your offering is not addressing a real need.
I once heard an anecdote about a guy who inherited some money and used it to start his own business. He hired some excellent people, rented nice office space, and got the best equipment available. Once he was all set up, he invited his old Business School professor to come visit. When the elderly professor was escorted into the new CEO's office he looked around at the artwork and state of the art computer setup.
"So what do you think of my new business," the CEO said.
"I don't know. Who are your customers," the professor asked.
"Well, we don't actually have any yet," said the CEO. "We're just starting up."
"Then you don't have a business yet," the professor said.
"Then what do you call all of this," the CEO said, waving his hand around the room.
"A lease," the old man said, with a sad little smile.