Two arguments are frequently made which I think have no merit.
1) "It interferes with readability of the articles." Sure it does, but that's a technical issue, and _if we wanted solutions_ we could deal with it through tehnical means. We could define better visual apparatus for references. One of the examples I keep pointing to is Laura Hillenbrand's book, "Seabiscuit: An American Legend," not because it's the only book that uses this style but because it is an extremely readable bestseller. It is densely cited, but there is not a single mark within the text. Instead, the references are placed at the end. They are indicated by chapter, page number, _and phrase_.
This would require modifications to work with Wikipedia, but that can be discussed.
Wikipedia has a unique _requirement_ for very dense references, _denser_ than those found in research papers or nonfiction books, so it is not surprising that traditional solutions are not perfect for Wikipedia, and that we will need to think of better approaches.
2) "If it appears in numerous textbooks it does not need a citation." This is silly. The problem is that there is no way the reader or anybody else can tell the difference between a sentence which lacks a reference _because somebody has checked_ to make sure that it appears in numerous textbooks, and a sentence which lacks a reference because_ someone just typed it in off the top of their head_. They look the same.
Even if someone goes over an article with a fine-toothed comb today and has made sure that none of the unreferenced material needs references, without any sort of markup apparatus there's no way anyone can tell a week later which portions of the text have been reviewed.
In other words, if we don't drop something into the article to leave a breadcrumb trail to where the fact was found, then any work we do in fact-checking will be wasted effort because it will be obsolete a week later. We need some kind of marker to where the fact was found. And the marker needs to be readily visible... at least to those interested in seeing it... so that anyone can see how carefully the article has been fact-checked and _which facts in it_ have been checked.
Finally, which is easier to do: check to make sure that a fact is contained in _three_ textbooks and say "good, it doesn't need a reference" and not put one in? or check to make sure it's in ''one'' textbook and cite the source?
I sometimes think that at least some people who object to citations do so because what they really want is to _establish themselves as authorities_ through social interaction with other page editors. That is, they want their Wikipedian colleagues to recognize _them_ as reliable sources, and agree that any fact inserted by [[User:Pantomath]] does not need a citation because everyone agrees that User:Pantomath knows everything.