"Do you think a different layout would make more sense?"
"Do you think it needs a line?"
I hear these questions a lot from students when I look through their portfolios. And my answer is always the same:
"I don't know. Go find out."
It's impossible to tell if a headline would clarify the ad until you've written tons and tons of headlines. There's no way to know if there's a better layout until you've done several so there's some comparison.
You can't talk theoretically about advertising. It's like saying, "If I had a killer headline and an awesome visual, would that make a good ad?"
If you have an ad you're not sure about, play with it. Write some headlines. Or taglines. Or body copy. You may find out it's exactly what you need. Or you may find out why it's perfect without them.
If you've laid it out one way and you're not 100% convinced, lay it out 10 different ways.
You've got to work. You cannot theorize.
Julia Cameron said it best:
"Art is not about thinking something up. It's about putting something down."