People say 'you don't want to pay 4 - 8 life' about library, but as a matter of fact some matchups do not care about your life total. Most significantly all the Show and Tell variants, against which you can easily fire library four times and you're gonna love it. To some degree against control decks you are fine with dropping to single digit life as well. Finally mirror and other grindy matchups are often more about CA than life total, and often times you find yourself in a position where entire game can turn around if you can pay 4 life to get removal and threat in same draw.
The 'no need to pay mana later' is a fine aspect of library, but additional draw is what makes this card huge (and why people don't just play mirri's guile instead). It is great against both storm and s'n't combo, very good against control, often useful against grindy mirror and only bad against decks like Canadian *****, against which you don't have mana to play with top anyway.

