Exactly, the state spent a lot of money (and/or resources) to train an engineer it would be best to keep him/her working as much as possible. Here we lay highly trained engineers off and they could be idle for several months. Lots of disadvantages to central planning, but realizing that it's expensive to keep highly educated resources idle they would try to maximize their usage.
" highly educated engineers were paid like 100 rubles/mo, which is slightly above minimum wage."
I think it's kind of hard to quantify based on salary alone in a system like that. People in more prestige positions like engineers & scientists had access to things that others might not have: a better apartment, better schools for their kids, shops that had a better supply of goods.
Why lay off people, when state can afford to keep them on minimum wage? and reap the benefit of their work (rockets, weapons, tanks, nukes, etc)