Since the forum mods decided to be difficult instead of remotely helpful, I'm remaking this here.
In the many "OMG - Detect Magic/Create Water/every cantrip is the devil!" thread, all have failed to take into account or otherwise ignored a single, interesting bit of rules pertaining to Detect Magic.PRD wrote:3rd Round: The strength and location of each aura. If the items or creatures bearing the auras are in line of sight, you can make Knowledge (arcana) skill checks to determine the school of magic involved in each. (Make one check per aura: DC 15 + spell level, or 15 + 1/2 caster level for a nonspell effect.) If the aura emanates from a magic item, you can attempt to identify its properties (see Spellcraft).
From what the spell states, unless the magic aura is being generated specifically from an item or from/by a creature, there is no way to actually discern what the spell is. There is just a magical aura of some order of magnitude. Or that's how I am reading it. A spell without a target (like the Image line) would only ever be identified as a Magic Aura of a strength relative to its caster level. You could never deduce what kind of magic it was. Similarly, if someone shot up a room with a fireball and you walked past and used detect magic, you could discern magic happened there of a particular strength due to lingering aura, but you would never know what the magic was.
Chew on that thought.
Yip, that's what I said in the forum, but people seemed to read right past it. Not that I particularly blame them for only giving the thread a cursory scan after it had become so unnecessarily mean-spirited.
So detect magic isn't unduly useful in rooting out illusion spells, unless the illusion spell is something like invisibility that completely masks the existence of something, and in that case the spell still requires 3 rounds of the invisible creature to stay in your cone before you suspect that the magical aura is illusion based. Detect magic doesn't gimp illusion; the fact that illusion isn't conjuration or transmutation gimps illusion. Illusionists are still better off than abjurers, evokers, and diviners. In fact by gimping detect magic, I'd say your really just punishing the weakest school, divination.
EDIT: The thing I posted in the other thread was your main point, that only auras that are 'born' by an item or creature get the full detect magical school treatment. Reading more into this thread, I see that it resulted in people also noticing that line of sight is required for the creature or item of an invisibility spell, and depending on your definition of line of sight, that should pretty much be impossible anyway. So my paragraph above is inaccurate in that not even invisibility should have its school of magic revealed. All the more reason that illusionists don't need house rules to protect them from this spell.