Client and developer arrangements are mutual, and not all clients require developers to respond within 3 hours on their day off. But, if your client does, you need to do it. Moreover, it is just important that you keep your word. If you tell your client you are going away Saturday and won't be able to answer emails, and you cleared this with them up front, there's not going to be any problem. But, if you told your client "Oh hey, don't worry: i'll keep the server running." and then you took off on a vacation without your smartphone and laptop, then that's a problem.
Many clients have had bad experiences with unresponsive developers. At TopTal, we strive to guarantee that they will receive the level of responsiveness desired.
I'm also a member of the netwokr, picking some work there, some through my own direct contacts. Generally, 'rules of the game' are always agreed with the client, when such rules aren't a fit for both sides, no need to engage in such work, as you wouldn't in the case where you find your own clients. Some are more, some other are less flexible, just like in every other place.