Aside from the obvious backstabocity, of course. I intend to make some kind of multiclass for my first insane solo, and from what I've read they're great... why? --- Irregardless, make my words, when you get down to brass stacks it doesn't take rocket appliances to get two birds stoned at once.
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.
They are the only legal multiclass specialist mage, so they are the only legal multiclass mage to get the bonus slots
Not quite. You can also have have a Fighter/Illusionist and Cleric/Illusionist.
Unless of course you meant that Illusionists are the only specialist mage with a multi-class option, in which case I'm sorry, it's a little late here so I probably misread it. --- My Little Pony is like a venereal disease: Really fun while it's happening and embarrassing to talk about afterwards http://www.elizabethwilkie.com.au