Remember, during the years when TOS was filmed, spray bottles of this type were relatively new and considered a pretty nifty idea (to butcher a Douglas Adams quote).
(Similar situation on Doctor Who and "bubble wrap". When they recorded the 'Ark in Space", bubble wrap had only recently hit the market, so not everyone and their hamster had seen it yet. Thus it was considered a clever idea to wrap an actor's hand in the stuff, tint it green and pretend it was alien tissue oveertaking some poor sod's body. Back then it had something of a "WOW!" factor.)