The deal is (I think) that Scala is a much quicker evolving language than say Ruby. Ruby 2.0 was rumored to be released the better part of a decade ago (anyone else remember Pickaxe 1.8, pre-Rails, saying that eventually parenthesis would no longer be optional in Ruby 2.0?).
Scala has developed new features at a pretty quick pace. By and large this doesn't mean breaking source compatibility. Most source seems to be forwards compatible, or require minimal effort.
The reason this comes up at all is because of Package Management. Where Ruby only needs to worry about a parser compatibility level, Scala has to worry about binary compatibility in packaging since it lives in Java's packaging ecosystem. Since Maven doesn't encode a binary version, breaking binary compatibility (say you added a new default argument to a method signature, that wouldn't otherwise break source compatibility, or you introduced a new superclass for a return value) means breaking your packages.
SBT gets around this by encoding the binary version (the Scala minor version) in the artifact name.
Some day this will all probably get sorted out.
It hasn't been much of a pain point for me personally. Most 3rd party binary packages are Java packages, which are binary compatible since Java 5(?). For the few Scala libs you might depend on, you might as likely be using a source dependency from Github, which means you don't need to worry since you're compiling it yourself, or most popular packages will have published versions with a new compiler pretty quickly.
The next time this will be an issue is when Scala 2.11.0 is released. And AFAIK that should introduce any breaking changes for source code at this point. So if you want to move to Scala 2.11.0 you either wait for package authors to cross-compile to it, or you build it yourself as a source dependency if you have access to the code.