From the MM description they're gaseous balls of light (essentially) and therefore are essentially useless for manipulating objects.
Though, technically small size with a reach of 5ft, they've got no melee attacks. Unless I'm missing something fairly major here, they could, by the RAW, be monks - and that would solve their lack of melee armament for AoOs in rogue builds - though, they're probably better off sticking with Monk.
With the LA of +4 or more, they'd be taking a big hit to their spellcasting ability as compared to other party casters.
Go Warlock - depending on how you picked your invocations, you could be pretty irritating.
I'd still personally probably take a Musteval (BoED) over a Lantern Archon.