The point is, most people normally playing the game don't obtain those power-summons/Limits. People normally playing the game* would probably find Sephy a moderate challenge. I found him moderately challenging before knowing about all these things. You might as well get all the Celestial Weapons and max your stats in Final Fantasy X, then say 'why was Jecht so easy?!1one'.

Furthermore, I detest long, expensive MP Summons and Limit Breaks, but to each their own.

*That is, without using a strategy guide or hunting these things down with previous knowledge of them.