If the battery is located inside of the car (such as under the rear seat) it is a no-brainer safety device that should be used. I bought my E320 used, and about a yr after purchase during a hard winter when I used the REST function at a grocery stop, I came out and the car would not start. Battery was dead. I discovered to my consternation that while the prior battery was a good brand, it did not have the plastic tube connected up....it was just laying there to the side. The battery that was in there was not capable of using the plastic tube--completely sealed.
I am sure there are debates on the board about this, but I went ahead and purchased a genuine MB battery and made sure the hose was connected properly. Have had battery for almost 1.5 years and it is holding at the rated voltage.....battery cost me about 110.00 bucks.
- Jon E320 210.055
77 Mustang 307,000 mi
63 T-Bird [fully restored]