I see customs in a different manner. For me customs is not so much a showcase of skill when you use them but more a showcase of skill and ingenuity when you make them.
A member who is able to do exceedingly good Earth customs, capturing the essence of the element and its main application and use in battle is undoubtedly a much skillful user of earth than one that can't.
Customs for me count not by power but by quality. A good custom is a custom that complements your own fighting skills, bio or fighting style. You make it for yourself to help your own style and not simply because you want a forbidden rank blast of fire.
People unfortunately don't do customs with a purpose or a focus. They generally do them in the pursuit of raw power or because they saw a similar custom they can't have and which they want. They simply do them without actually stopping to think if its useful, good or actually has a purpose.
I agree and disagree with Zen in terms of what customs say about your skill.
The Customs you make, showcase your knowledge and skill in understanding and knowing the elements, the arts and how you apply them. The customs you have access from others to don't say a thing or say very very little.
However, to be a referenced, excellent fighter you don't need customs and in that I agree with him.