Any weapon or shield stolen in battle can be equipped by any character, as long as their menu is up when Sneak takes place.

Weapons stolen in battle also have their spell data mangled. As a result, for the duration of the battle, a stolen weapon is usable as an item. The spell cast is based on the type of stat bonuses or penalties the weapon gives. Weapons with no stat changes cast Disrupt, which will instantly kill an enemy, ignoring the boss bit.

For a full list, see: FF4 Sneak Weapon Spells, by Aexoden

If a stolen Drain Spear is used as an item, the resulting spell is the Call spell that Searchers/Summoners/etc use to re-summon an enemy: it looks for an empty slot and adds a monster of the second type. This can potentially create a grind fight, but in many cases will simply crash the game.