Thank you for the opportunity to help you with your question!
The estate tax is interesting in that it is a fairly small tax (0.7% of federal receipts in 2013).
The purpose of the estate and gift taxes is to reduce the inequality of inheritances. We're not taxing the dead - you can't take it with you, as they say - we're taxing their beneficiaries. If you're inheriting over $5.4 million (the tax-free amount), it may put you in a different social and economic position from someone who has to earn his/her wealth. On a nationwide level this could lead to the concentration of wealth in family dynasties, and undue power to lazy heirs, rather than the American ideal of social mobility and meritocracy.
And frankly, inheritances of over $5.4 million affect so few people who have so much money that it's hard to get too upset for them. I suspect it's by far the largest amount of money you can ever receive (by not even earning it!) tax-free
Please let me know if you need any clarification. I'm always happy to answer your questions.