It's very strange for me that a lot of customers do not know the difference between a web developer and a slicer.
Every time I find something a customer asked strange, I ask them about their jobs, and quickly find out that I have absolutely no idea what it is that they are doing (apart from a very high level overview sometimes). It's not strange, it's perfectly normal.
Unless of course your customers are web designers ;)
Of course, a web developer MUST know html and css but I don't think that he should slice the PSD and do all the work a css guru can do...
The line between web development and web design is fuzzy at best, where one ends and the other begins is debatable. There is nothing wrong in having some basic web / graphics design skills, and slicing a PSD is a very basic skill, something that you can pick up in hours.
Whether it's a skill that you must use or not is up to you, your specific situation and your relations with your customers. Still use and know are different things, even if you are never again asked to slice a PSD, why not be able to do it?