Some westerners speak little Thai because they don't feel the need to learn. I know one guy who still says Fuket instead of Phuket after 20 years.
I taught myself from a book, which means I can read well but sound rather academic, like written Thai, when speaking. I can write my name and address in Thai but my work writing is all in English. My staff all speak excellent English, but I have never spoken English with my driver or the better half's family.
Is it useful to at least speak Thai? Absolutely, Thais love that you made the effort. Even bar girls.
When I was about to get my permanent residence at immigration, I had a chat in Thai with a policewoman at the small ceremony (just a photo for their records). She said to me rather sadly, "Do you know," sweeping her arm to include the rest of the room, "You are the only one here who can speak Thai?"
Speak Thai, no matter the mistakes, and see them light up. 😀