And the thing about Summons is that their power is limited. Sure Elementals can grow as the Caster's INT and Elemental Affinity increases, but even then that is limited and Demons don't grow with the caster at all (cept they get more HP, that's it). Sasha's Revolver will keep on growing stronger as she levels up Dex, Int, and Focus Score (Wisdom + Focus Talent). 

So a summoner would have to spend time and risk (this varies) contracting new beings (demons are the hardest to contract, and sometimes they don't listen to you, even if they are loyal, they show a bit more rowdiness then elementals do and may sometimes act depending upon how they are feeling). Like a demon that is all about shredding opponents apart may not act as a shield to the summoner. (like Galkyr is). 

Although to be fair, about the blindness (part of which is inspired by the Gamer Manwha) I think Lucian and Mike have somewhat clear paths now (Mike learning Seishin no Ken/Divination Skills and going from there and Silas learning more sword techniques from Genji) is also a bit clear. 

So I guess there isn't as much blindness as we think? I dunno XD