From:runescape.com

In our latest Summoning Development Diary, we are going over what it takes to test and balance a skill so that it can fit in with the rest of the game.

While Summoning isn't just combat-related, its effect on combat will be significant. As such, a large part of this Development Diary's focus is on balancing combat, and the effect it will have on the Combat level. The Diary also covers where to pitch the skill in terms of difficulty, cost and training speed.