The point in no having customs before sannin rank is this:
Most of you use only your customs on battles and have no idea about the actual Naruto jutsus. I have seen it many times in senseis tests. People who i thought they were stronger than me, they don't stand a chance against other people in a non customs fights. I dislike that fact and as you all know i am allergic to unfair things.
Second point: You all rush to make customs and some of you have already reached the limit of 30.why? Cause depending on your fights now and your opponents' ranks & abilities at the moment you make customs jutsus that will help you, now.
Soon you will all reach 30 and then you won't be able to create anything for future battles and future opponents. Take for example me, i have submitted 13 jutsus, i am one of the people with the less jutsus made and although i have more of them (devlin's training, zanji's co-creations etc) i never use them. I have won Hells & Zanji using only the normal jutsus before and i am equally strong when i don't use customs. why? cause i know very very well to use the normal naruto jutsus and i know what they can do.
Allowing you to make customs when you reach sannin rank means:
That you have proven that you are able to win a non customs battle against someone that has higher rank than you (meaning that you are ready to request to be a sensei), that you have gained enough experience (missions) to actually see what you miss in battles and of course what is the thing you want to be your specialty, that you have developed strategy & tactics (since you are fighting with someone of higher rank than you)
You just don't want to lose the fun of making costums without seeing how "back stabbing" this is to you. You have for example 20 techniques you are so eager to keep them you don't see what you earn using this system.
For example someone who is a Jounin: He can be a Sannin in 25 days,getting trained under 4 different senseis and learn: 4th element, Battle training, KGs training, fuuinjutsu, advanced taijutsu and ninjutsu. All of them. Not to mention the experience he/she will get from the missions.
Bottom line: i don't agree with this.