Yes .. You need those Unbound Runes for Cultivating your Summons, which also gives you additional Power, not that big but still, additional Power .. You can earn more Arena Coins, just accomplish missions everyday and you'll be fine .. Saving for those things that you're not even sure when they will come is like, saving foods for rainy days while right now you're dying from starvation .. But saving or keeping coins and some sort is not bad, just know how to balance the considerations and importance of them for you ..

Yeah bro! I'm at Summoning Jutsu Lv6 now, and currently cultivating my Serpent, Lightning Tiger and Dog of Hell .. I'm at the 4th Element which is the Mountain Forces and the runes needed ranges from 80 for first hex upto 180 for the last hex .. So for sure, the last 2 forces which is the Lake and Earth Forces will require more runes for each hex :| hahahaha.. that's why we need to play Arena everyday and to be honest, i'm a little bit lazy playing Arena for points xD