Ninety five percent of human genomic DNA does not code for proteins or functional RNA molecules, and is frequently referred to as "junk" or "selfish" DNA. The vast majority of this noncoding DNA has no documented role in the cell. However, according to recent analyses, three quarters of the human genome is transcriptionally active. We discuss whether the expression of non-coding genomic sequences is valuable for the cell or if it is a second-hand "junk" because of the incompleteness in transcriptional machinery organization and functioning. Introns constitute a major fraction of the noncoding DNA, representing over 40% of mammalian genomes. They are ambivalent elements that cause several problems and at the same time bring benefits to their host cells. There is a strong correspondence between the average length of introns and the size of the genome. Here we review the latest summary statistics on human introns, the evolution of introns in mammals, and the distribution of genes that encode functional RNAs within introns. We also suggest that splicing is an important filter for organisms with large genomes, serving to distinguish between functional mRNAs and arbitrary RNA transcripts generated from random loci.