The mining guilds will set up shop selling component X for Y credits.
In that case, I don't think storage space is the showstopper for treating engineers as a side activity. It's the RNG mechanic.
OK here's the "rewarding" part of the Engineer add-on.
Let's say you're out and about in your mining ship looking for the final piece to complete a recipe to modify your C4 Cannon of Doom on your BH Death Ship.
You find it .. hurray ... dash back to dock and realise you forgot to put cargo slots in advance on your death ship - cos you know, it's a death ship, not a cargo ship.
Now what ?
You can't transfer to your death ship as you have cargo in your hold .. and your stored ship has no space .. and sadly your mining ship can't hold a C4 Cannon of Doom either (to modify that and transfer it over via sell/buy)
You're out of luck.
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To give Pecisk some credit I suppose he is right in some aspects that Engineers should have been a side order to deal with as and when.
Sadly the implementation of it by FD pretty much killed that off.
Multiple layers of RNG (finding / rolling) combined with no storage to easily swap between ships makes this "side" dish rather tasteless.
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I'd suggest it's a side activity as you can do it when you want, just like pretty much any other aspect of the game.
Of course ... but once you start down the path you have to complete it .. you can't easily abandon mid way though for reasons pointed out above.
It may well have been designed as a side activity but the implementation makes it a primary.