One of the biggest cons is to be told that marketing is a separate entity apart from sales. Total and utter nonsense. Try defining your marketing objectives that have nothing to do with sales and you have a business plan that will go down the drain.
A Marketing Plan should have its main objectives directly related to the sales plan. First and foremost marketing is all about sales. Marketing objectives should always be developed with sales objectives in mind. Never, ever, forget that in the end the success of every marketing plan ever written is ultimately measured by sales.
So how do you write, develop and implement a Marketing Plan? Simple, call! Now!
Sure you could have a go yourself, maybe even do a great job but there is nothing like an objective outside viewpoint.
Remember that your time is best spent making sure the Marketing Plan is implemented correctly, not writing it! You definitely have better things to do with your time.