the way i was told is that as long as you are not operating in any state's waters for more than 60 consecutive days, you do not have to be registered in that state, i.e. state numbers or stickers. this is the case for il and mo but you will get hassled a lot more even if you do display name and hailing port on the stern of the boat. the documentation aspect allows you to not have to pay sales tax, so this is the benefit to answer your question. i went ahead and paid the "in-lieu-of-tax" for missouri just so i could display the sticker and not get hassled(without sticker on old boat i was stopped 7 times in 10 days with just doc. info on stern of boat). some states can register your boat with no in-lieu-of-tax such as delaware still i believe. those that trailer your boats, do not have to be registered in any state to be documented and be legal. this was as of last year and i believe to still be accurate.