I think the utility of religious systems can be determined by looking at the reason for their continuity:
Abrahamic faiths tended to continue via wars and forced conversions. Dharmic religions such as Buddhism tended to continue because adherents saw such immense value in the teachings that they spread them even despite resistance from authorities.
They have both existed for millenia but I see eastern faiths as more useful because they were rarely forced on their adherents.
As to your point on designing your own system, I think it's just hard to design something as complex as a religious system well. Instead of reinventing the wheel, people just choose the best among the time-tested methods. For me Buddhism seems to best fit the bill. If you want examples of people designing their own Religions, look at Crowley's Thelema, or for more philosophy-focused look at Theosophy.
Everyone has a philosophical or theological code, but most people just use what they are given. Even people who don't explicitly have a system still implicitly use a value system to guide their decisions.
As to your point on designing your own system, I think it's just hard to design something as complex as a religious system well. Instead of reinventing the wheel, people just choose the best among the time-tested methods. For me Buddhism seems to best fit the bill. If you want examples of people designing their own Religions, look at Crowley's Thelema, or for more philosophy-focused look at Theosophy.
Everyone has a philosophical or theological code, but most people just use what they are given. Even people who don't explicitly have a system still implicitly use a value system to guide their decisions.