I think it is a big deal, now if you are a long time sprint customer you probably won't miss, appreciate or even care for something you never actually had before. It didn't bother me at all until I got the OG Evo 4G, and being able to talk on the phone while having data was pretty convenient when needed. Now for someone that don't really talk on their phone that often then I can see it not being that important but for someone like me who talks on the phone quite a bit plus send and receive emails all day long as well as various other things that may require data, its now a feature I refuse to live without.
They can still market voice and data at the same time since neither the sprint or verizon can do it, not even over lte as Robert was saying. From a marketing stand point I think that its a huge advantage that att will definitely exploit.