It would be like someone wanting to be a Christian group that insists David Koresh was a prophet of God. That would be cult, even if they call themselves Christian. Just because Mormonism has stood the test of some time, does not negate its roots.
Also, you should remember that God supposedly told Joseph Smith to start the LDS because all the other Christian denominations had gone astray. So Joseph Smith basically called all other forms of Christianity cults or false religions. Now the LDS flip their lids when mainstream Christians do not accept them??? The LDS may be Smithians, but they are not Christians.