The most important issue in alternative Splicing, in my opinion, is whether splice variants are due to splicing errors (= junk RNA) or whether they reflect real biologically relevant alternative splicing.

Unfortunately, this view is not shared by the majority of scientists who work in this field. They are convinced that the vast majority of splice variant transcripts represent real examples of regulation and the main task is to document the extent of alternative splicing and characterize the various mechanisms.

I've written a lot about this topic over the years (see the list of posts at the bottom of this page). The two most important issues are: (1) the frequency of splicing errors and whether it can account for the splice variants and (2) the number of well-established, genuine, examples of biologically relevant alternative splicing and whether that's consistent with the claims.

I managed to post a summary of the data on the accuracy of splicing on the Intron article on Wikipedia and I urge you to take a look at it before it disappears. The bottom line is that splicing is not terribly accurate so we expect to detect a fairly high level of incorrectly spliced transcripts whenever we look at a collection of RNAs from a particular cell line. The expected number of mispliced transcripts is well within the concentrations of 'alternatively spliced' transcripts reported in most studies.