They're not completely useless, because they can move a negotiation along a step e.g. by ensuring exclusivity, or agreeing on language in subsequent negotiations. PR side effects aren't empty either: investor confidence can keep a struggling firm alive; or for manufacturers, having a LOI in hand may be beneficial to your supply chain relationships.
A word of warning though.
Anyone who thinks a LOI violation isn't actionable hasn't met bigco lawyers. I was taught long ago to basically treat LOIs as purchase orders. They are hard to make completely nonbinding. Whether an LOI, a MOU, or a HOA; all such preliminary agreements fall into basically the same category of potential legal time bombs. An imperfectly drafted LOI may be treated as billable by some parties, with all the actionable response this implies if you withdraw from the arrangement.
So someone asking you to sign a LOI or similar preliminary agreement who suggests "Oh it's just a letter of intent" is probably taking you for a chump. Do not sign without very good legal advice, and (unless you are an excellent lawyer) never attempt to write one yourself.
