The key point is that Perot was leading both Clinton and Bush in the polls prior to dropping out. That changed after he dropped out and re-entered.
As I said, it's a highly imperfect analogy. But I fear that voters could be turned off by her voluntary failure to complete the term for which she was elected. Could make her look wishy washy and flighty, which is how Perot came off.
Obama didn't finish his term for which he was elected. Hillary didn't finish her term for which she was elected. Dole didn't finish his term for which he was elected. Reagan would not have finished his term for which he was elected he he won the '68 nomination.
I could go on and on.