“Secure” is a relative term.
Is my house secure when I lock the screen door?
Of course it is. But if something matters to you, you take steps to secure it.
Is my house secure when I lock the screen door?
That depends. Are you sitting in the living room with a shotgun? Do you have a big, mean dog?
If you don't care about what's in your email, then it doesn't matter if you use Yahoo! or Google or MSN. I have accounts on several of them that I use for throwaway stuff.
Business stuff and personal communications with friends goes to my mail server. Is is secure? Well, that goes back to your point. What is secure?
It's locked up in my house. When I'm not there, it's got dogs and alarms. It is very unlikely that someone could get in, it is nigh impossible that someone could get in, break into my mail server and get out without me knowing about it.
That kind of thing happens all the time with Yahoo!.
So, is my email secure. Opinions vary, but I can state with certainty that it's more secure than Yahoo!.