I am not a lawyer, this is not legal advice.
It is true that in the USA, anyone can file suit for any reason. Some few obviously frivolous lawsuits are thrown out before trial, but most do not.
A waiver may be so well-written that you will definitely prevail in court. But you could be sued by a non-member who happens to wander in off the street and trips walking in your door, or a person who just doesn't think your waiver is all that.
In either case, you still have to deal with the expense of responding to the lawsuit, which involves retaining an attorney at the very least. Even if you win, that does not mean you will escape without paying a lot of money and possibly having your reputation damaged. If you lose, you may find yourself in owing money your business cannot repay. Depending upon how your business is structured, you could find yourself personally liable for any judgment, and even worse, depending on the type of judgment, you might not even be able to bankrupt out of it. You could find your home and real property with liens placed on it, your day job (if you have another source of income) salary garnished.
That's why it's generally a good idea to have liability insurance.
Here are some links I found just using Google. I am not endorsing any of them, I have no idea if they are useful in any way. Just FYI.
http://www.summitamerica-ins.com/products/martial_arts_insurance/martial_arts_insurance.asp
http://www.fdean.com/martial_arts_insurance.html
http://www.nationwide.com/general-liability-martial-arts-hb.jsp
http://www.martialartsinsurance.com/Pages/default.aspx