For characters with prepared spells, it also serves another purpose--the ability to get some use out of a spell that would otherwise be rendered useless by the circumstances.
For instance, a fighter/wizard packs Command Undead because the party lacks a cleric and it's very good for dealing with undead mooks. When the party decides to invade the orc stronghold, and doesn't run into undead, he can still get some use out of that 2nd level slot. Or, the fighter/wizard is packing Feeblemind but, when he reaches the end of the cave complex, he runs into a priest and a barbarian. Not too much call for Feeblemind there. So he uses Arcane Strike. The same is true for the fire evoker/fighter who runs into fire elementals. His scorching Rays are much better as Arcane Strikes than as nothing.
Also, note that it has very good synergy with reach and combat reflexes, expert tactician, and other ways of getting extra attacks. In the best case, a fighter/Wizard/Eldritch Knight/Spellsword with Blink and haste active, combat reflexes, and Expert Tactician could make 6 attacks in his normal round and then 5 attacks of opportunity (when all the people he tripped stand up for instance). If he had cleave or Great Cleave as well, it could add up to even more attacks. If he used Arcane Strike on one of his 9th level spells, that could theoretically add up to more than 117d4 damage. (A good deal better than using that Wierd spell against the undead creatures he's facing wouldn't you say?) Of course, that's a best case scenario and it's unlikely to actually happen but it does show the potential usefulness of the feat on a more realistic scale.