Point of source units treat the water in the same manner but only at the faucet being treated... so you must consider cooking, drinking, ice cubes, bathing (particularly with smaller children as arsenic is cumulative).
In my opinion and I have no knowledge of resale values... a home with an R.O. unit would give me piece of mind that my entire home is treated... regardless.
Feel free to contact me with any additional questions or clarification...
Here is a link for a flyer with more info on arsenic in water. It includes a phone number for state officials that you can call to ask questions.