Here's a video of a cover for The New Yorker created on an iPhone.
Consider how much detail the artist puts into what is eventually obscured. He makes a nice little crosswalk, a cue, and a couple taxis. Then covers them up with a hot dog stand and silhouettes in the foreground. That doesn't mean he was wasting his time.
I've seen a lot of portfolio students resist experimentation with tag lines, headlines, certain visuals and even media because they didn't think they'd be necessary. They have an idea of what the ad should be, so they stop working as soon as all their requirements are met.
The truth is you won't know if your ad needs a tag line until you've spent some serious time coming up with a sheet of the best lines you can write. And as much as you love that visual you came up with, you'll never know if it's the best until you try to come up with at least three that are even better.
Put in the time and effort to paint that crosswalk and those taxis. Who cares if they're covered up? It doesn't mean you wasted your time. It only makes the finished piece better.