The main purpose for tablet usage is to run apps, surf the web and check email. An 8GB and certainly a 16GB is ample for those needs.
I tethered my Asus TF101 to my phone and watched the Euro footy on it when I was out and about waiting for the kids to finish their activities.
I have an SD card in my TF101 but it isn't used. Firstly because I don't listen to music much, and if I do I use Spotify, Vevo or Tune Radio. Movies, well I watch those on the big lounge telly with the family not on a tablet.
You also can't move apps onto the SD card on the TF101, might be the same on the Nexus 7.
If I need to copy files on or off the Tablet, I use either Google Drive or Dropbox, again no need for an SD card, as when I logon to my PC the files are synced and almost immediately available.
All those people that won't buy it because it hasn't got an SD card, must have large music/movie collections they wish to put on it.
If you don't have those needs then you will not miss an SD card slot at all.
I certainly intend to buy the 10" version when it comes out later this year as the kids and misses squabble to use the one we have at the mo.