Let me start of with an analogy:
Does a bank teller need to know how to read and write? Possibly they do, in any event if they do not, they wont be especially good bank tellers.
Same thing here, if you don't know scripting (as opposed to MEL) its like you wouldn't know how to read or write rigging procedures. There are several reasons for this. But mostly the most prominent reason is that using maya and scripting maya is equivalent. If you know how to do things in the GUI you know how to script it (mostly).
Rigging Artist really does Knows the COMPLETE MEL.
I don't think anybody does know complete anything. So no not complete. But certainly if you know Maya you know how to script it. Adequate level is enough.
In the end, it's a question of efficiency. sometimes a script is the easiest way to do things. Sometimes the only sane way to do things. And in reality this is not really hard at all, just a bit scary if you have no experience in writing instructions.
You dont need to know how to script, but then again you dont need to know how to rig either. So you dont need anything some thinsg are just universally regarded as a good skill. Scripting is one such skill, it is especially valuable if you intend to work with computers.