Noob Question: Why do I want 61.5?

I'm thinking of switching to Dish (from DirecTV) and I think I've got most of questions answered, but one I can't seem to fully grasp is all the hubub surrounding 61.5.

I'm in the Detroit DMA, and I read somewhere that my locals are now 'available' on 61.5.

From what I can pickup on the forum, that appears to be a good thing. But I can't seem to figure out why!

It seems like that I would need two dishes (1000.2 and one to pickup 61.5) to do that - is it really worth it? Are they in MPEG 4 on 61.5 and MPEG 2 on 129 - is that the big deal? I plan on using an OTA antenna for locals anyway (to pickup subchannels), so I likely won't even use the Dish locals. I realize though that I still have to buy them to pickup my local RSN.

Any insight/advice would be appreciated!
Thanks...