They say the church stands or falls on the testimony of Joseph Smith in the Sacred Grove meeting God.
If Joseph Smith is wrong, then there's no point in being a mormon.
In the remote possibility that Joseph Smith was right, there is still no point in being a mormon because of how completely dysfunctional that church is or has become.
Joseph Smith may have been true, though it seems very unlikely, and even if he was true - throughout history and even in present-day-lds-mormoism there is so much dysfunction and stupidity that there really is no point to go there anymore.
This is a very easy and simple answer to understand. My personal problem is that I've always got this nagging thought process in the back of my mind reminding me of the LDS church.