This plugin communicates with the Skype application in the background to perform its work, so it's necessary to have Skype installed and running.
If it really needs the non-free skype application to work properly, I would say it's a clear candidate for "contrib".
-- Dipl.-Phys. Fabian Greffrath Ruhr-Universität Bochum Lehrstuhl für Energieanlagen und Energieprozesstechnik (LEAT) Universitätsstr. 150, IB 3/134 D-44780 Bochum Telefon: +49 (0)234 / 32-26334 Fax: +49 (0)234 / 32-14227 E-Mail: greffrath@leat.ruhr-uni-bochum.de