I think I've done that before in games with enemy tiers. It's a pretty useful tactic to help sell the progression in strength. I don't like the soulbound weapons having that effect, since it would overvalue it. It and summoning are supposed to be optional elements that you enter into. 

I'm fine with the summonable monsters being something you can encounter in the wild. As well as some of them being intelligent and capable of being talked to and stuff.