My main point is that a tarball is a neutral starting ground from which you can generate all other things: packages and installers. So providing your software as a simple tar file is convenient for everyone down the line.
And that is something I won't argue with.
Is the linux version delayed? It doesn't appear under the SteamOS + Linux tab on steam.
They intend on waiting to polish up the Windows and Mac versions first, which really makes a lot of sense. I don't know about you, but I'm one of the few gamers that doesn't duel-boot or have some sort of fancy hardware pass through. i doubt there are too many of us, so really a Linux version is just a nice thing Larian is doing. Well, that and it gets the game of SteamOS, which probably isn't going anywhere.
[edit] Luckily it seems to work pretty well in Wine, though when I tried playing Co-op with my brother it did crash. We haven't had a chance to trouble shoot the issue, and only tried once.