Lets pretend for a moment that you had zero spirit and 100000 spell power.
Life tap would read: Life Tap - Now converts [ 1.5 x 0 + 1490 ] health into [1490 + 100000 * 0.5 ] mana.
Expanding this: Life Tap - Now converts 1490 health into 51490 mana.
And we're bitching... why?