I purchased my 1st home last year and a few months ago I hired a company to do a cleaning of the chimney. Since I have a gas boiler they said the chimney was clean and didn't need a cleaning, but they tried to sell me on the idea of getting a chimney liner. They argued that gases could get through the tiles and/or loose tiles could fall and obstruct the chimney. This being a 50+ years old house I can see a point to what they said. I did not hire them to do it, but found DYI liner kits for a fraction of the cost (they quoted $2000, I found a kit for $600 ******). I think I could install the kit myself and save money, but I'm unsure if I really need it. I have (visually) inspected the chimney and it looks fine (no missing tiles, etc). A friend of mine that flips houses told me if the chimney is fine then I should not mess with it...but of course her view is from the point of view of someone that wants to minimize expenses and make a profit...
I too want to minimize expenses, but if it's going to prevent tiles from caving-in or CO from getting in the house then I guess I should do it.