Not entirely correct. There are Nas's that have tons of features, but not all of them do. As I said, he doesn't have to go all out just because he wants to be able to share files over his network.
So instead of thinking "am I going to use all the functions of a NAS" (which is like think "am I going to use all the functions of a cell phone"), he should be thinking about what functions he wants and then get a NAS that suits his needs.
I do not see the justification in spending money on a network attached storage device and then skimping on it. It's an investment, and not usually a cheap one. If you are looking to buy one I would advise you but a 'proper' NAS. Whether or not you use all the features now, at least you have the option to use them in the future as and when need be.
Using your phone example, people buy a phone to make and receive calls, text message, check their email and access the net. An iPhone does so much more, a hell of a lot more with all the apps available etc, and hence people purchase it - not because they need all of the features right now, but because they know it's all available to them and they have the flexibility to do so much more with the phone as they use it in the future.
It seems the Op wants to share files, and enable DLNA for media streaming for now. But what if he later decides on an iTunes server, or time machine backups etc? At least this way he has the ability and the flexibility to do so, and it makes his NAS last as long as possible.
I purchased my NAS originally with the intention on using it for storage/backups only. Since then I've started using DLNA to stream movies to all my devices, iTunes server, and more. In fact I'm also now looking to install security cameras around the house and driveway - at least I know my current NAS has the ability to run these all for me, even though I had no intention of using cameras when I first purchased the NAS.
I guess it all comes down to not only what the OP wants right now, but what he may want to use in the near future. Why limit yourself? Of course, the current budget will also play a big deciding factor in his decision!