I see situations where this is extremely annoying. Let's say you build a t4 in your base after some air fight happened. The t4 finishes, the 60 engies around the t4 now just go and randomly split up to go hunt down 150 mass wrecks while you were busy microing a fight and you come back to pure chaos organization in your base. Same sort of thing could happen if you have engies waiting to help assist the t3 pgen once your t3 engie is finished from your air fac.
Not to mention some maps have shit trees that are worth nothing so you never go reclaim them. If they exist in your base and you keep any idle engies in base for bp then these engies go off and waste their time on awful trees that provide near zero reclaim and now you need to go and remember to move them back to where you intended them to be.