I can think of very few disadvantages to making principal only payments on your mortgage if you can afford to do so. You might want to take a few extra steps to ensure they are posted properly to your account.
If you can, I suggest you go to your bank personally to make the principal only payment. That way you can make sure it is applied properly. If not possible, make sure you send in the payment certified, return receipt and check your statements carefully after each payment is made.
Also, another thing to consider are investments that might yield more dividends than the principal only payment.
Check with your CPA or financial planner for more options available to you. You want to make sure your money is working for you in the best way possible.