THere are people that genuinely need the economy when they do 20k a year but also there is the dark side of the type of people that money saving and false economy is their favourite hobby so they buy an Omega(value for money), stick an lpg(usually a bad cheap and nasty system), and then run the car until it stops due to lack of maintenance. So there is the LPG car-tool-workhorse that is being used a lot but treated well and on the other side there is the heap of junk of MrTight pockets. I don't want any of them to be honest. Of course thats my personal opinion
It's just like any other car, when you go to inspect it look at the overall condition, how clean it is, check all the receipts and old MOTs etc. Oh and check the installation quality of the dual-fuel system, on a V6 it should be SGi which costs in the region of £2k to have installed professionally. With that in mind, I don't think many people are going to spend that kind of money and then stop maintaining the car so it breaks down before they've even recouped the cost of the installation let alone saved any money!
A properly installed and maintained dual-fuel system actually reduces wear on the engine, as when it's running on propane you don't get anywhere near the issues with bore wash and oil dilution that you get with petrol. My engine is on 170k and I recently changed the cam covers, the cams looked good as new and my breathers have never been blocked any time I cleaned them out. Other than a failed crank sensor (which happens to all V6s eventually it seems) and a stripped spark plug thread I've never had any real problems with mine.
There are dogs out there that are dual-fuel, and there are dogs that are completely standard. Personally I wouldn't consider the dual-fuel aspect to be a good indicator of the level of maintenance. People are prepared to pay a fair bit more for a dual-fuel Omega that's been looked after, so I'd suspect the cheapskates are actually more inclined to go for the bargain bucket petrol-only models and run them into the ground, which is why you see so many abused and battered early models barely scraping a few hundred quid at resale.