In my subfield of computer science I observe that workshop papers often are not thoroughly reviewed. It even appears (attention, possibly controversial assumption incoming) that workshop-paper acceptances are used to increase the number of participants at the conference, because authors of accepted workshop papers are required to register for the conference. Moreover, workshop organizers are often desparate to get enough submissions in order not to be forced to cancel the workshop. Hence, they are happy to accept submissions even if they do not completely fulfil certain quality standards.
Speaking about the quality of the papers, they often do not contain rigorous evaluations, hence, the presented results possibly are not really reliable (please note, that, of course, there are many terrific workshop papers out there!). I have a feeling that these papers would not get accepted at a (good) conference due to lack of maturity etc. I am aware that (especially in computer science) the publication path is often like this: early/initial work in progress is submitted to a workshop → if the work gets more mature it is sent to conferences → after some more work it is probably sent to a journal.
My question is: Given the above circumstances, should workshop papers be cited?
Since the quality (hence, the results) of such papers is often lacking, I ask myself whether it's a good idea to make a reference to it. One further thought is that references are often used to show that something has already been done or addressed. Therefore, the reference is used to argue that the problem addressed by the workshop paper should not be addressed anymore which is probably a wrong assumption if we assume that the work done in the workshop paper is sub-par.
Don't cite bad quality work as evidence. Judge the quality yourself. Only cite things if you think they are of a sufficient quality to demonstrate the claim you want to reference. Don't rely on peer review (whether in a journal or conference). Lots of poor quality work gets into journals, too.
Work in progress and early results do not equal bad quality. Something that is still in progress, or hasn't been rigorously demonstrated yet, may still be worth reporting. This doesn't make a bad quality paper, as long as the paper is honest about the interpretation of what is presented. Falsely claiming that early results are definitive is bad, but "Our early results seem to show this, but we aren't sure yet" can be a perfectly good paper. There is nothing wrong with citing such results, as long as you similarly are honest about what (and how much) they show.
In short, There is no reason for a blanket refusal to cite conference papers. Such a practice might make you miss important evidence. Do consider the source and the quality of the work, and try to cite the strongest source(s) when possible. This will often (but not always) mean preferring a journal article.
I'm assuming you are thinking of cases where you are searching for evidence in a particular area and can choose what to cite. Of course you also have an obligation to cite anything your work relies on or uses, regardless of the source.