A good marketer with a mediocre product is still going to get the nod.
Yes, if one is selling Amway, but a good agent is going to be seeking something she can sell to a publisher.
Nothing changes the basic rule that you have to write words that are worth buying.
I had the worst query in the world. I knew it, the editors knew it. But once my writing got up to snuff that changed everything.
It took me two years, 25 rewrites on chapter one and 2-3 full rewrites of the whole book (on a MANUAL typewriter, yet), but I sold that first book.
Absolutely yes, TERRIBLE books get into print. I know for a fact that my writing is miles better than several undeserving hacks on the NYT list, but it's a part of the industry that s**t happens. You just keep writing.
I can drop the hint that helped me to sell that first one:
Make sure your opening is such that it will distract a tired, bleary-eyed slush pile reader from a subway full of muggers and cause her to miss her stop home.
When that happened to me she took my 50 pages and synopsis up the food chain to the senior editor.
Thankfully they ignored my horrible query letter and bought the book.
And just so you know--after more than 20 novels sold I STILL can't write a good query!