Documentation for the method containerForSource
http://api.kde.org/4.x-api/kdelibs-apidocs/plasma/html/classPlasma_1_1DataEngine.html#a623f9becf3d4a464a64ce0ee2f8fa70d
tells:

This method should not be used if possible.

What is the reason? Efficiency, future compatibility, or something else?

I looked at source code of several dataengines (e.g. microblog, rtm), they all use this method notwithstanding.

Could you enlighten me on this topic?

--
Yuri

I would recommend asking the Plasma Developers, either on #plasma on Freenode, or by emailing plasma@kde.org which is a mailing list.