Well, I may be wrong (I don't have an ARC but do use swype and have followed it since the omnia 2 release) but I thought the arc came with a sony "gesture" keyboard, not actual "swype", some don't realise that "Swype" is a brand name and a company that has recently been aquired by Nuance, thus changing how you can get it on android, I too had to get the beta to run it on my commando. I have tried 15 or so different keyboards after migrating to android from my omnia 2 (the first phone ever to have swipe) and have found that there is no real substitute for it available, not T9trace, not BK8 not any of it, so I use the beta. Now I'm not for sure on why they took swype off the android market but still give it to us for free, but I do know that if swype was native on your phone and you downloaded beta from the market it caused force close and now they ask you some device info before just letting you load it up so that may be why.
Good luck with your keyboard,
John