New2grammarTo me, having both just sounds weird. Can someone convince me otherwise?
Think of the difference in meaning if you leave out 'should':
I believe that she is leaving. (It seems to me that she is about to leave. I see her making preparations to leave. I conclude that she is leaving.)
I believe that she left. (It seems to me that she went out the door. I look around, and I don't see her anymore. She has gone, as far as I can tell.)
I believe she should leave. (There was an emergency call. Someone in her family is hurt. The best thing for her to do now, in my opinion, is for her to leave -- to go to them and help and comfort them. It seems to me advisable for her to leave. -- Alternately: She is drunk. She is embarrassing herself and others. She needs to go home and sleep it off. It seems to me advisable that she leave.)
I believe is the it-seems-to-me element; should is the it-is-advisable element. The it-is-advisable element is missing from the first two sentences.
CJ