William Blair analyst Jason Ader says he can't sit "idly by" when a stock like Pure Storage (PSTG) is offering what he views as "exceptional" risk/reward proposition at current share levels. The analyst's downside scenario is $9.45, or roughly where the stock sits now, versus a "conservative" upside scenario of $14 and "optimistic" upside scenario of $20. These scenarios represent appreciation potential of 43% and 95%, respectively, the analyst notes. Pure is a much better asset than Nimble Storage (NMBL), which recently agreed to be acquired by HP Enterprise (HPE), Ader tells investors in a research note. The deal shows that outside buyers continue to see value in the storage space, the analyst contends. Ader keeps an Outperform rating on Pure Storage.