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If you get out a lot regularly into a variety of environments, the travel cover is absolutely worth it, in my opinion, for a couple of reasons.
When I bought my ARB 50 almost four years ago, I asked the vendor if the cover was worth it. He kind of shrugged and said "Eh...I guess."
I got the cover and put it on within the first couple days of owning the fridge. It's been on all this time until just last week; almost four years. I was re-organizing the van interior and did a major cleaning, including removing the fridge. I wanted to wash the inside with a bleach solution then rinse it with a hose, so took off the cover. My fridge looked brand new; all plastic clean and free of abrasion from other gear knocking around or from daily use.
When parked with the fridge exposed to direct sun on warm days--which I try to avoid but is inevitable at times--the insulating travel cover most definitely helps moderate temp swings. If it's the same cover I got four years ago, it's rugged, has a good exterior pocket to hold the AC cord, manual, or whatever, and is lined with reflective material.
Just the fact that it provides a thermal layer and air space will help keep your fridge cooler, cycling less, and from swinging towards ambient temps as quickly.
If I was keeping my ARB fridge in a workshop or home most of the time when not adventuring, and only going out the occasional weekend or vacation, I might not be as concerned about having a cover. My fridge lives in my van full-time, though, and I'm always throwing lumber or my bike or other gear in there; or working somewhere with both rear doors open and the fridge exposed to the weather. So, for me, it is absolutely worth having.
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