Another benefit of short sentences is that it adds pep to your copy. People don't like to be bored while reading, and especially while they are reading advertisements. Don't make your advertisement, long and droning. So don't do that to your reader. Keep your ad copy short enough so that they are not immediately turned off by its length.
White space makes the ad copy look shorter and more appealing.
Even if your copy is long, it will look much more inviting to read than a solid block of text. The reader will feel like they are quickly flowing through your copy, compelling them to reader further. Your copy will seem more personal and more likely to appeal to your customer. Make sure that the ad copy provides some sort of clear benefit. Money making program.