I thought it was a nice balance to the fact that you are guaranteed to know what items a mob has. It is an illusion to you, but it can still whup you (just like the phantasmal killer).

Edited to add...another reason is that it makes dealing with illusionary gear much simpler (someone mentioned when "you" grab it that it disappears, which is possible, but much more complex when you're trying to find out who is a valid "you", and who would cause it to disappear).