One might say that Marx is like Newton, describing/discovering many things and setting a foundation for their field. Saying "we shouldn't read Newton because his stuff is old" or that his ideas are wrong simply because they are old is ludicrous. Both of them probably had things they got wrong, sure, and newer theory corrects this, but they still set the foundations.

While one might not read Newton directly in school, so for some Marxist theory it is too (see Elementary Principles of Philosophy teaching DiaMat), but Marxs books that haven't been superseded in this way should still be read.