Always use a letter of intent to present your preliminary offer to a seller. This will save you writing time, legal fees, and will be easily read and understood by the seller and any other interested parties who must be made aware of your intention to purchase the subject property.
We find exactly the opposite to be true. If you were to go to an unknown area, would you want a map of the area? Well that is exactly what the LOI is: a map of the major deal points. In today's busy world it is important to be sure two parties are going down the same road and in the same direction before spending too much time working out all the details of a transaction.