You know, you need to know what is best for you and the dog. As long as agility is fun for you and your dog, why not?
i am so tired of people saying "You can't do this or that because your dog is not bred for it."
Guess what, you can! You can train a draft horse in classical dressage and teach them everything a high profile classical dressage horse can do. It will never look as graceful and they will never be able to compete to the same level as a high profile dressage horse but you can do it and thats the point.
And by the way, over here there are draft horse races, so they can race!
You can have a big GSD do agility, you may not be as fast, it may not be as graceful but you can do it.
As long as YOU and YOUR DOG have fun doing it, do it!
Some people just don't understand that it's not all about competing. Sometimes people do things just because they are fun.