Options are good - you can either make mods, find mods, or trade for them.
There's no more guarantee that a given trader will sell a mod you want than there is you'll find it or a schematic thru looting, so isn't it nice to have more than one way to get lucky?
I disagree on the magazines. They don't need to be unbalanced and stupidly rewarding. Sell the ones you don't want to a trader. You can use some of them to buff a skill and craft stuff early, which is pretty powerful already. Buffing a good skill for 30 minutes is already pretty solid.