arts-1.0.1-6 contains the *.la libtool scripts in /usr/lib, which are only needed when linking against the libraries, so could be moved to the -devel package.
No. KDE uses ltdlopen() to load libraries dynamically, which doesn't work if the *.la files aren't installed. Try it for yourself - rm -f /usr/lib/*.la /usr/lib/kde3/*.la and watch KDE breaking.