I wrote a letter to Dr. Sam Griffiths-Jones, one of the founders of miRBase for this case. He said:
All the evidence in miRBase points at mature sequences, not precursors. You therefore can't get experimentally validated precursors out of miRBase (or from anywhere else I know). The precursor sequences are essentially only indirectly validated by study of the mature sequences. For example, if reads from a miRNA sequencing experiment map to only one location in the genome, and both potential 5' and 3' mature sequences are supported, then the precursor sequence is implicitly validated.
reference to the paper claiming 50% validation?
Yes, the article is here. http://bioinformatics.oxfordjournals.org/content/26/18/2226.full Or just google it with the sentence.
thanks, I'm gonna read it.