whats so great about getting silent assasin how come everybody is so interested in it???

Well the only incentive for actually getting the Silent Assassin Rank is that the player reieves more money and can therefore spend more money on upgrades.

However I think the main reason that most people like to get Silent Assassin is because they like the thrill of being able to do it. The exhilaration of killing the mark without anyones knowledge, being able to walk out of the level past security without hint of guilt or remorse, and without suspicion knowing that you have just performed the Perfect Murder...

Or that might just be me.