I'd rather think Riku would be considered twilight due to the end of Reverse Rebirth. And I don't think any keyblade has one purpose. A keyblade is all about the heart. We may come to find out that a keyblade is just the physical representation of one's heart. If a person's heart is strong enough he will have a keyblade, this power isn't limited to a person being good or evil.
Another theory could be that the person with the strongest heart takes control of the keyblade of a particular world. Say the strongest person representing light would take control of the keyblade representing the light world. Anther for darkness, another for twilight, and another for present. This could be backed up by that incident where Riku stole the keyblade from Sora, I can't exactly remember but when he stole the keyblade did it not look like the kingdom key? Then when Sora took the keyblade back Riku summoned a dark keyblade, now I ask myself if Riku could summon another keyblade, why couldn't Sora? This may be due to that previous theory of keyblades representing different worlds, if at one time Riku could of been the Keyblade Master then maybe with his extra strength in heart he was able to take back the kingdom key. But when Sora grew even stronger, with Ansem's evil influence he was able to summon the keyblade of darkness. Thus Riku may very well of been able to use his heart to summon two different keyblades. But now Riku has taken the twilight path, this his heart is a mixture of light and dark. If BHK's heart is also the same, then who's heart would be stronger and who could gain control of the keyblade representing the twilight world?
This is of course all theorys, and I could be completely wrong, but it does make some sense, so maybe anyone can summon a keyblade, or there may only be one keyblade per world, who knows....