I still don't see where there is a real problem at all. At least half of the illusion spells in the core rules are combat spells or spells that have applications in combat. In combat, detect magic isn't going to help most of the time (try using detect magic on color spray), and in the cases where it might help, you'd have to be desperate. In the case of the vast majority of illusion spells in the game, knowing that something is an illusion isn't going to make a difference. What it really comes down to is that detect magic can determine the illusory nature of a few effects at low-level that are really only intended to slow someone down in the first place.
Here's my own list of uses for illusions from the core rules that are NOT busted by detect magic:
Ghost Sound - This is mainly intended to be a momentary distraction. It can even be used to communicate over long distances.
Color Spray - This is an instantaneous effect.
Silent Image - Use it as a wall to block opponents' line of sight in combat for a regroup.
Ventriloquism - This is a distraction spell. Forcing someone to cast detect magic distracts them.
Blur - This is a combat spell.
Hypnotic Pattern - This is a combat spell. It can be used outside of combat to distract masses of people, but fascinated people aren't able to cast detect magic.
Invisibility - Who in their right mind turns invisible and stands around in the same spot for three rounds when a spellcaster is nearby?
Magic Mouth - This is used to impart information. Who cares if detect magic can tell its an illusion?
Minor Image - That wall you used to cast at 1st-level is now a big ogre who bellows threateningly. A distraction is all it needs to be.
Mirror Image - This is a combat spell. Even if one character can tell which images are illusions, they shift from round to round and you have to concentrate on detect magic to keep it up.
Displacement - Think of this as Greater Blur.
Invisibility Sphere - See Invisibility.
Major Image - What was once an ogre is now a fire-breathing dragon. A distraction is all it needs to be.
Greater Invisibility - See Invisibility.
Phantasmal Killer - This is an instantaneous effect.
Rainbow Pattern - See Hypnotic Pattern.
Shadow Conjuration - This might be an instantaneous effect. Even if it isn't and the subject disbelieves, it is still 1/5 real.
Dream - This spell sends a long-distance message to someone who is sleeping (i.e. incapable of casting detect magic).
Nightmare - See Dream. Anyone who is sleeping isn't casting detect magic.
Persistent Image - Suddenly that fire-breathing dragon can move around without you directing it. While your foe is casting detect magic, you are killing him with other spells.
Shadow Evocation - Many evocations are instantaneous, particularly the ones you'll probably end up using with this spell. Even if they aren't, they are still 1/5 real.
Mislead - See Invisibility, only this time the subject has no reason at all to believe you are invisible in the first place.
Permanent Image - That dragon can guard your stronghold. How many people are going to be willing to get within 60 ft of it in the first place?
Programmed Image - Often used to convey information under specific circumstances, in which case it doesn't matter if you know its an illusion.
Shadow Walk - This is a fast travel spell.
Mass Invisibility - See Invisibility.
Project Image - While a caster wastes three rounds determining the image is an illusion, the image has cast a fireball, a lightning bolt, and a cone of cold for good measure all while the REAL caster is hiding safely 230+ ft away.
Greater Shadow Conjuration - See Shadow Conjuration.
Simulacrum - Okay, so you've figured out my minion is an illusion. He's still a pretty darn good minion.
Scintillating Pattern - See Hypnotic Pattern.
Greater Shadow Evocation - See Shadow Evocation.
Shades - See Shadow Conjuration, only on steroids.
Weird - This is an instantaneous effect.
A gnome illusionist using nothing but the core rules can own this game with illusions.
This thread, busted.
