In the SBS for volume 65, someone asked Oda why he gave Luffy the relatively weak power of turning into rubber over something cooler and stronger. His response was:
"Ahaha, that is true. It must seem like a mystery when you consider all the other strong and cool abilities. The answer is simple. I have picked the most ridiculous ability. If the protagonist were the typical strong guy, I doubt I could continue on with this for long. No matter how serious the story gets, Luffy is there to stretch and inflate. He always give me a chance to fool around. That's the kind of manga I wanted to write."