In Breaking Bad series 3, Gus organises a whole high-tech lab so Walt can make large volumes of his high quality crystal meth. The lab comes with a competent and reliable assistant.
But Walt contrives a feeble excuse to dump the assistant and bring back the somewhat unreliable Jesse. The excuse that he can't work together because of technical competence isn't very convincing and there is no obvious personality clash (in fact early on Walt overcomes his early suspicions and the two then seem to get on well).
So what is motivating Walt to dump a reliable assistant for an unreliable partner?