Use Your Illusion
Figments are good for causing distraction and worry in opponents, which can be of great use in combat, particularly with something strong and relatively stupid, where denying it even 1 round of attacks while your allies attack can be a most valuable use of a spell.
There was a Rules of the Game column on the Wizards site where it was suggested, much as someone pointed out here, that the idea of "interaction" be extended to include taking a standard action to study something carefully, usually with it having to be in close range.
As others have stated, it's up to the DM to determine whether a certain type of interaction makes it impossible to fail a save, or provides a hefty penalty for believability (or penalty for lack of believability). This means that the player would do well to specify the qualities of the illusion in a way that increases believability. For instance, illusory opponents can be made to specifically miss on every attack (even if a near miss) to avoid triggering a save when a blow should have landed. In many cases a good choice is a translucent opponent, for instance one made to look like a ghost. If the illusion is of a creature that is believable as something incorporeal, then if an opponent lands a hit on the "ghost" the DM may grant a penalty to the save for believability (or if normally inclined to make the save automatic with a "corporeal-seeming" illusion, at least allow a normal save). Be careful with that example, however, as someone may have a ghosttouch weapon or, as happened to me once, the opponents decided that since they couldn't hope to hit incorporeal creatures with much success, they decided to more or less ignore my illusory allies even though they believed in them.
In the case of an illusion that looks like a Blade Barrier type effect, you might tell the DM that you want to make sure that the density of blades is such that it looks difficult but not impossible to leap through unscathed. The downside is that may encourage an effort where you'd rather maximize the chance that the foe stays back and doesn't try to cross, but the upside is that the foe, if he fails the save, may more realistically believe he made it through and not dismiss the illusion out of hand. The relative value of keeping the foe on the other side and keeping the foe believing the illusion of course depends on the situation, so both strategies have their place.
Here are a list of some things I've made illusions of in combat with my Illusionist/Druid:
* A wall of water that tricked magmins into spending precious time going around it so that we could catch up and clobber them before they disappeared into a magma flow.
* Ghosts and apparitions of various kinds (see above)
* An illusion of my animal companion or an ally so that the opponent is worried about what is real and what is illusory
* An illusory hedge that my allies knew was fake and could fire through with arrows while opponents had to make saves to realize it was an illusion and see through it to target us.
* Various things that seemed extra tasty or vulnerable to the foe so it would be encouraged to waste time attacking the illusion instead of me
The main benefit, in the end, is to make opponents waste time. The more you play D&D and higher level your campaigns become, the more you realize that time (i.e. rounds in combat) is the most precious resource of all. The question eventually becomes not "What can I do that's useful?" but rather "What's the the most useful thing I can do this round knowing that the opponent is going to do the most useful thing it can this round and the fight probably won't last more than 5-10 rounds?" Sometimes, a Figment is a poor choice, but in other cases the best use of my time is to make my opponent waste his (while my Dire Wolf animal companion chomps and trips him, of course).
Cheers,
MC