If you're running Biker Lists, static gunlines or Vulkan, then Vanilla is better. Anything else, there's not realyl a reason to not go for BA or SW.
I don't know... even here SW's can create a pretty static shooty list if so inclined. Three long fangs + razorback spam, etc. They could just sit in their deployment zone and blaze away.
As for me, I wont be giving in to the red thirst, but mainly for financial reasons. If you're a space marine player and you want to assault the enemy, I can't think of any cooler army to do it with. So many new toys, and even though the Stormraven is an ugly model, it would be tempting.
Excellent firepower, decently armored, fast skimmer, that can transport a full squad AND a dreadnought into close combat where upon they can assault the turn they disembark. What's not to like about that (aside from having to purchase one or more ugly expensive models)?
Provided you steer clear of BA specific things like special characters and Death Company, you can use their codex to represent anyone.
Good idea. I've often thought about doing something like this. One it allows you to play the with the cool toys of that codex without needing to purchase all new models. This seems like a good balance to me, and isn't much different than taking Vulkan or Pedro, etc. for a vanilla marine army other than Salamanders and Crimson Fists and saying he "counts as" your chapter's special character ________________ (insert appropriate name).