This question is being followed by good number of people which fascinated me to throw some light on this.
JEE MAIN spot round is nothing but a simple way to fill up the vacant seats in NITs which are not filled up during counseling.
If you don’t get desired seat during your counseling then you may wait to gamble on spot round. Mind the word GAMBLE. You can’t be sure if you will definitely get a decent branch in good NITs. Students normally rely on spot round only when they have nothing to lose and only option left with him/her is either try spot round or go for other colleges or taking drop.
Hope it helped 🙂