1. Why I don't want it.
There are a lot of people who didn't play Vanilla WoW and who are used to "convenience" of modern wow.
My biggest fear about bundling Classic into the same subscription as Live wow is that when Classic comes out those people will decide to take a look at it and after some time go like:
"What is this sh..t, Blizz? Why am I out of mana after 2 mobs?" Or "C'mon Blizz, why is finding a group taking so long?"
And knowing Blizzard, they might give in.
Of course, some will argue that bundling Classic into the same subscription as Live will mutually benefit both games. Really, though? Will people go beyond "taking a look" to recall why he/she quit the game (or to recall how "inconvenient" game was back in old days in case of retail players). Don't think so.
2. Why Blizzard don't want it.
Answer - cash shop. They constantly report that the amount of money they make from an average player is constantly growing. To sell mounts, tokens, server transfers etc. is impossible on Classic servers, otherwise it would ruin them. So it's in Blizzard's interest to do everything possible to prevent moving the player base from Live to Classic, but encourage them to move from Classic to Live.
Simplest solution to this would be to make Live Wow players to pay on top of their subscription in order to play Classic, but at the same time to make it free for Classic wow players to play Live wow.