The Marathon Plus is pretty flat resistant, but it does still get flats. I wouldn't advise running over goat head thorns with reckless abandon and not removing them when you pull off the pavement at stops.
Just a note to say that the flat liners themselves can cause abrasion flat in the tubes, so be super careful in installing them smoothly especially at the ends of the strips. I do't run them myself, choosing a tire with a built in belt, but I like the idea of them better than an overbuilt tire like the Marathon Plus. I care about the weight, but perhaps even more I like a nice supple sidewall and the road feel and lively ride that it gives.
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I use to live in the Mojave Desert of California where goatheads breed far faster than rabbits, there was times during certain seasons I would be riding with a dozen thorns attached to the tire going around and around...but I was using Specialized Armadillo All Condition tires and NEVER got a flat in the 7 years I was there using those tires, before those tires I tried all sorts of tires and they all failed to stop those thorns. Now if Specialized Armadillos can protect the tube from goatheads I really sure the Marathon Plus tires can survive since they're built to be more flat resistant then the Armadillos were. Also don't forget I mentioned the Marathon Plus not the Marathon regular tire, the Plus has 5mm of flat resistance built into it than the regular Marathon which has only 3mm.
I'm aware of the flat liners causing the abrasion problem you spoke of, but I found two ways to deal with that, one is to sand down one edge that will be against the tube so that it's really thin; the other stunt is to use liberal amounts of baby powder. I never had a problem with abrasion flats doing that stuff.
Weight of the Marathon Plus does concern me, but a ton of people use that model for touring so I guess they don't seem to care, but I think it would bother me. The tires that came on my touring bike were Kenda Drumlin tires, those darn things weighed 1,600 grams a piece, what all that weight did for the tire I have no clue because they didn't last long, about 2,500 miles but only about 1,000 of that was loaded, and I got one flat.