So... you've worked out exactly how much spell power it gives, and can compare that number to other trinkets, at least 6 of which (other than IotDS) give strictly more and (generally) other benefits to boot. It seems to me that you've answered your own question, so what exactly did you want responses about?
I have noticed no one has mentioned [Majestic Dragon Figurine] at all. With Fel Armor currently granting 30% of my spirit as spellpower and the new glyph in 3.1, Glyph of Life Tap (20% of my spirit to spellpower when I tap), is this trinket truly undeserving of any mention? In the current patch it is approximately 56 spellpower and in 3.1 it will be 90 spellpower. Is this a trinket worth considering while I wait for [Illustration of the Dragon Soul]?