The problem is that with most of the big bads we just know too much about them. Sure, they are scary but now come in easy to manage packages. With instruction manuals.
The fundamental attribute of any monster must be its unknown quantity. Think about the first ever Alien. Just one monster, creeping around the dark and scaring the bejeesus out of everyone. JUST ONE.
When a bad-guy is BADASS, you don't need more than one of them.
That's why I'm so excited about the new series, I'll bring you a new level of fear. The demons out of Indian mythology don't fit in boxes. They take many forms. Some haunt graveyards, others search the battlefields at night, feeding on teh dead, others are heroes in their own right and some, a few, challenge the gods. The Indian mythology doesn't play by Western ideas of good and evil. Look at Kali, the goddess of killers and the most bloodthirsty being in all creation whose dance of frenzy almost destroyed the universe. BUT she's the greatest defence we have against the forces of darkness because she's BADDER than the rest.
The same applies to rakshasas. By bringing in a 'new' mythos I can scrap all the things we know and love but (lets face it) have become blase. Can you honestly say that you don't roll your eyes a little when you see another vampire book? Be honest. Thought so.
The unknown. It's the source of all our fears. Lets have it back, shall we?