I feel strongly that going forward, only the core standard library should stay in the scala._ namespace (or any of its sub-namespaces).
It's unfortunate that modules like scala-parser-combinators are already in that namespace, but I think that is a historical accident, stemming from the standard library's bad old kitchen-sink days, and doesn't qualify as a precedent for our current choices. (We should be looking to move those modules out of scala._ space, even.)
tpolecat has already covered most of the main reasons for my opinion. I second his points.
Also, it hasn't been explicitly been pointed out, perhaps because it's something we all already know, but scala._ is imported by default into every Scala program. This is a major reason it is so privileged and a major reason we should be very conservative about introducing anything into it.
By contrast, in Java, only java.lang.* is automatically imported, not all of java.*. Things are different in Scala because we have nested packages and Java doesn't, but that difference means we need to be more conservative than Java about namespace real estate, not more liberal.