When trading futures spreads, butterflies etc. you're taking a view on the (future) shape of the curve. Technical traders generally fade any 'humps' in the curve, while longer-term traders may take a fundamental view on how the curve should change if certain conditions emerge / cease to exist.
You can always chart 2 calendars to create the implied butterfly - this is better than creating the implied from the outrights, but obviously still not as 'safe' as an exchange traded product.
Note - if you're just getting into the world of spreads etc. I suggest you have a look at more liquid / less volatile markets rather than the product you mentioned in your next post.
STIRS are good for starting out.