Writing for sales and marketing purposes is not primarily about "good writing." It's about human behavior, the psychology of sales and using words that sell your product or service.
Think about it. It's not organizations that buy your products or services. It's not government entities. It's not anonymous faces. It's people.
People do business with people they know and trust: this is as true when writing as when you're talking to someone in person. How do you begin to establish that trust? By writing in a form and language that they can relate to quickly, that make them feel they're having a cozy chat with you over a cup of coffee.
Here's the key: when you read, you hear the words in your head. If your writing is conversational, the reader will hear it as if you're having a conversation with them and they'll relate on a deeper level.
However, when you speak, you don't always honor the rules of grammar. Often you don't speak in full sentences.
Dangling participles, contractions, and sentences ending with prepositions: these are some of the things that were drilled into us at school as forbidden. And in good literary writing, that's correct. But in marketing, breaking those rules is often a step in the direction of conversational copy - because that's how we speak.
Or course, the language also has to be appropriate to the audience and the product or service. But in the end it doesn't matter how flowery, elegant or beautifully poetic your language, if it doesn't affect your audience and make them feel that comfortable with you, chances are it won't work.
Here's a really famous example of bad grammar that works. The Rolling Stones had a major hit called "I can't get no satisfaction." Would that have been as appealing if they'd followed the rules of good grammar and written, "I can't get any satisfaction?" I don't think so because good grammar destroys the rhythm that made the title catchy to begin with.
Don't get me wrong. I'm not suggesting you throw away your grammar books, or never open Strunk and White again. What I am suggesting is that hanging onto the rules of good grammar out of principle may make your marketing materials less attractive to your ideal clients than you'd like them to be. Be flexible. Be willing to overlook the rules in favor of the effect, where appropriate.
On the other hand, don't let the belief that it has to be conversational lead to sloppy writing, simply because you think you have a good reason not to be precise. That doesn't work either.
Remember the old adage that there's an exception to every rule? Create your own exceptions! And watch how response shifts.