Depends what you call waste! Everything has a price, in this instance to use a standardised protocol, there's a bit of overhead. Obviously for just reading or writing 1 register you pay dearly. In the normal scheme of things using Modbus, you'd be reading a number of registers each time some the percentage overhead would drop. Consider sending one byte via tcp/ip - I'm sure the numbers would look a lot worse!

Also, consider reliability - even for a few bytes its wise to have a checksum - for small payloads this also adds a substantial percentage overhead.