The "long ending of Mark", as it is called, is an actual textual issue. The fact is, the oldest manuscripts simply do not contain the material found in Mark 16:9-20. In fact, in some later manuscripts, there is even a verse 21!
So, what are we to do with the long ending? Throw it out? Totally accept it and ignore the historical/textual issue?
I don't think either is acceptable.
We should accept Mark 16:9-20 (21) with the understanding that there are some real questions about it. Therefore we should be careful not to base any teaching soley on the material found in that section. Thankfully, I can't think of much in Mark 16:9-20 that is not found somewhere else in the New Testament. Thus, in my mind, it is almost a non-issue. Something I don't think about unless asked to.
Just like thinking critically does not mean to think bad about something/one, textual criticism is not about being critical of a text. New Testament textual criticism is simply "the study of the original wording of the New Testament" ("New Testament Textual Criticism: A Concise Guide. By: David A. Black, pg.7). In this context, criticism refers to the careful study of something (Oxford: evaluation, appraisal, assessment, review, etc). If we are talking about thinking critically, that is carefully examining our thinking on a subject. If we are talking about New Testament textual criticism, we are talking about the careful study of the manuscript evidence.