educate me please

how important are ARB's or teather straps for ERF? if they are so important why are seats made w/o them? Esp newly made seats such as MR65? why don't you see them on very many infant car seats? is it a deal breaker for you?

I don't think there's a lot (any?) real life statistical data about whether anti-rebound devices are beneficial. There have been quite a few seats that have had them over the years (including a Britax and Combi infant seat with tethers, Britax and Combi and Fisher Price seats with anti-rebound bars, Britax and Sunshine Kids and the old Sit N Stroll could be RF tethered). Most (all?) larger RF seats in Sweden can RF tether, though, and presumably it's a safety benefit in their crash data. Australian infant seats have anti-rebound bars and tethers that top tether, but they only keep kids RF to 9 months or so, typically.
Anyway, any RF seat is just so incredibly safe it doesn't really matter if it has that additional feature. It would come down to personal preference, I think. I chose a seat that RF tethers for my own kids, but also used an infant seat (Safeseat1/Snugride 32) up to 18 months, and used the Scenera for a spare, and would buy the MR 65 now for my 3 yo if she were short enough, because I'd rather have her RF even if the seat can't tether.
Click on the 'here' link in my siggy and look for 'top tether, rear facing' to get a little more deep info on the topic, too.