I believe he was referring to the multiclass elf, option 2.
The I/T will make a better bomber if you are willing to give up necromantic spells (with Skull Trap being the best necro spell) - extra spells at levels 1-4 come in quite handy.
While a mage/thief offers a lot of potential for trickery, I've always preferred to separate the classes if I'm going for power over roleplay. A M/T has low hp, is nearly worthless in melee, if it is a gnome I/T it cannot use the gnome/halfling-only helm, it cannot use longbows, and is often vulnerable if used as a scout/decoy (and therefore targeted). It's better to use it strictly as a mage with better shooting ability. If you are scouting for traps, be prepared to run like a rabbit, and make sure the M/T is wearing a ring of Free Action (in case you run into a spellcaster). Don't use it to sneak attack or backstab on a regular basis. When going into melee with this character, which should only be in cases of extreme need and/or ambush, Mirror Image is necessary. Also keep a MI handy when scouting, as you could be in big trouble if you come unstealthed.
In sum, a M/T is a good support/mage character, but should be kept out of melee combat just like a mage. It is difficult to balance the scouting role (particularly if scouting for traps and therefore still visible) with the need to avoid being the enemy's target.