Yes, after you petrify an enemy you're forced to resolve the XP bonus on the spot. Why are you not then waiting until you need the XP boost before picking up IMAWAL?
One is a flat XP boost, and one is an XP multiplier; their function is actually very different in practice.
The flat XP boost is primarily used on low-level monsters that you can 1-hit, allowing you to "ding" mid-combat against a stronger foe. Because you're in combat, expending mana or health on other things is untenable, so you'll be attacking level 1-3 monsters. The flat bonus works well since the mana cost can be expended well
ahead of time and the monster left in a convenient place for harvest. As a multiplier, the actual gains from IMAWAL are marginal (if not net zero) when used against such low-level monsters.
The purpose of IMAWAL is the multiply the value of high-gain targets which award large amounts of bonus XP. In other words, you're probably taking down a monster several levels higher than you and will be expending all of your renewable resources. The point isn't to "ding" from this kill (it's highly unlikely you're simultaneously fighting other monsters, so the ding would just go to waste), but rather just to accrue a large number of XP for the long run.
There are a few maps it's phenomenal on - Hexx Ruins in particular
I mentioned this in my original post. Any scenario with plentiful no-XP enemies is going to be a major boon for IMAWAL, but these are the exception and not the rule.