The advantage is that you get more abilities sooner, making it easier to clear rooms.
The problem is that there's a level cap limiting the number of abilities you can earn in a given adventure, and most adventures are designed to give you enough XP to be at or close to the level cap by the final boss. Taking Smart ensures that you'll hit the level cap earlier, and takes up an ability slot to do it. If you have the Inspire Talent, levelling up heals you, so hitting the level cap early means you won't get that healing when you're most likely to need it. So, in most cases, taking Smart... isn't.