Interesting idea but there are basically no reasons except for Shirahoshi being a powerful weapon for her to join the Straw Hats. If it was only because of the power, half of the CP9 would be already in the crew and that wouldn´t make any sense. And splitting her tail would also mean that her body will slowly change and that wouldn´t really be nice to watch, I´ve had enough ex-mermaids in Water 7. Not every new character with different high leveled powers or with different appearance will join the SH crew.