What's the implications of not having this replaced? Is it an expensive job?
When a thermostat does not open the engine will overheat which can result is catastrophic damage to its internal components. A thermostat that does not close will cause performance issues, and poor heat output. If you have symptoms of the engine overheating or no heat output, you should check that the thermostat is working. Here is how to check if your thermostat is functioning. With a cold engine, remove the radiator cap, start the engine and watch the coolant in the radiator. If the thermostat is closed, as it should be on a cold engine, there would be no movement of the coolant. As the engine temperature increases the thermostat should open and you will see movement to the coolant. If this is not the case, then it is likely that the thermostat needs replacing.
Mine appears to be shot too as my temp gauge sits between 50 & 70 all the time unless I'm sat in traffic for a while and then it will shoot up over 90 and the fans will kick in and cool things down.. Luckily I've got a friend who's a mechanic who can fit a new one for me, I've seen they are about £33 new on eBay so I'll be getting one next week.
This week I replaced the discs.. The previous owner didn't replace them even though they were below the minimum on the MOT advisory for 4 years running.. The bolt that held the front left disc on was pretty much threaded and welded on! It took a good 2 hours to force it off.. Now I can see why the previous owner and garage skipped replacing them.. When I look the pads off, I was surprised to see that only the edges of the pads were getting worn as the center of the pads.. about 80% of the surface wasn't making contact with the discs because they were so worn that there was a 4mm lip around the edge and this was the only part which made contact.. So I'm surprised I've been able to brake at all really!
What I'm saying to you is.. You shouldn't be asking things like.. what are the implications.. you should be getting it done regardless.. These parts are there for a reason.. if it stops functioning.. something else will also follow suit.