They are the only legal multiclass specialist mage, so they are the only legal multiclass mage to get the bonus slots. Gnomes also get the racial bonus to saving throws for every 3.5 points of Con.
However, the loss of Horrid Wilting is a huge drawback depending on how you play. If you play it as a backstabbing thief who uses his mage side to buff him and make him a better thief, then the loss of Horrid Wilting is no big deal. If however you play it as a mage with thief support, then the loss of Horrid Wilting does matter.