There are two most common ideas on sigils:
1.) Anyone can 'charge' a sigil, even one they did not make themselves. OK, fine. But how do you know what that sigil really is, in that case? What if the person makes one for one reason, then releases the image online labeled for a different purpose? The people who believe that even if there is no actual subconscious connection, that a sigil can still be charged and used for its original purpose would tell you that it can only be charged for the purpose of its creator, and the one end result its creator itended.
2.) A sigil without an actual subconscious connection is meaningless in the mind of the person attempting to use it. It must have a meaning to the individual in order to put it to use. Many believe the best way to form this connection is to create the sigil -- that way it is completely personal, with every bit of nuance the creator of the sigil intends.