The "HD enabling fee" is not to enable your DVR to receive HD. It's to enable you DVR to even work in the first place if you DO NOT BUY ANY HD.
Buy an HD package, no enabling fee.
Don't buy an HD package, pay the enabling fee, and your receiver will work for SD and also for HD you bring in with an OTA antenna. You only have one OTA tuner so you'll only be able to record one HD program at a time (but you can record two more SD channels simultaneously via the satellite tuners).
Don't buy an HD package, don't pay the enabling fee, and you own (rent) a very large doorstop.
I don't think you can just pay the enabling fee any not buy ANY programming packages. In order to get one of these 622's or 722's you have to agree to 18 months of programming (even the cheapest package qualifies - HD not required). I'm not sure what would happen after that 18 months ... could you continue on paying ONLY the enabling fee? For that you'd get one OTA tuner and recording capabilities (although no guide data unless you purchased locals - I don't think you can purchase just locals with not some SD package either, but I didn't investigate that part).