It's perfectly legal according to the rules.
Is it just "rude" because it's considered as a low-level serve and using it against someone means that you're not taking the game seriously? If this is so, I strongly believe that people should get this notion out of their mind.
Just to make one thing clear, I don't use, nor have I ever used an underhand serve during a match.
Now, as to why I think an underhanded serve should be considered as just another normal serve and whoever uses it should not be criticized:
1. Your serve sucks, therefore for your second serve, it's easier for you to get the serve in if you just use an underhanded serve. I have seen people just throwing the ball up and tapping it in for their second serve...and underhanded serve can't be worse than that if not better.
2. Throwing in an underhand-serve once in awhile to throw your opponent off.
3. This is the main reason why I think an underhanded-serve deserves more credit:
An underhand-serve that has a strong side-spin or slice component to it is extremely difficult to return effectively. First off, the side-spin will make the ball bounce off to one side. Secondly, the under-spin + side-spin together makes it extremely hard for the returner to just slam the serve back; you don't get any pace to work off of AND you have to pay attention to the spin so that you wont frame your shot. This serve would of course bounce fairly short, thus drawing the returner right into no-man's land after he returns the serve.
It's definitely not that easy to control an underhanded serve with heavy spin, especially to the preciseness of a moderate-pace kick-serve. If anything, anyone who uses an underhanded serve against me during a match will definitely catch my attention.
Is it just "rude" because it's considered as a low-level serve and using it against someone means that you're not taking the game seriously? If this is so, I strongly believe that people should get this notion out of their mind.
Just to make one thing clear, I don't use, nor have I ever used an underhand serve during a match.
Now, as to why I think an underhanded serve should be considered as just another normal serve and whoever uses it should not be criticized:
1. Your serve sucks, therefore for your second serve, it's easier for you to get the serve in if you just use an underhanded serve. I have seen people just throwing the ball up and tapping it in for their second serve...and underhanded serve can't be worse than that if not better.
2. Throwing in an underhand-serve once in awhile to throw your opponent off.
3. This is the main reason why I think an underhanded-serve deserves more credit:
An underhand-serve that has a strong side-spin or slice component to it is extremely difficult to return effectively. First off, the side-spin will make the ball bounce off to one side. Secondly, the under-spin + side-spin together makes it extremely hard for the returner to just slam the serve back; you don't get any pace to work off of AND you have to pay attention to the spin so that you wont frame your shot. This serve would of course bounce fairly short, thus drawing the returner right into no-man's land after he returns the serve.
It's definitely not that easy to control an underhanded serve with heavy spin, especially to the preciseness of a moderate-pace kick-serve. If anything, anyone who uses an underhanded serve against me during a match will definitely catch my attention.