I have, many times so far! Aquascaping competitions are a great way to share your work with a wider audience.
I wouldn't say that building an aquascape for a competition limits creativity, quite the opposite. The winning aquarium should be something "never seen before". The problem is that sometimes such aquascapes remain unrecognized.
There are many factors that need to be fulfilled to score high, from the idea to the realization.
I actually don't do aquascapes exclusively for competitions, something that I think would do well with judges... Most important for me is to enjoy it the process and to have a tank that is easy to maintain.