The higher parlours give you better options for upgrading and making more money, but for full increase you have to offer/employ better girls (according to Sex Appeal condition of building).
When you have finished all the pimp courses and unlocked all possible parlours, you should find some good girls (80+, better divine ones with 95+). You should also keep in mind to protect your parlours, because higher income increases also number of uninvited guests (here a good gun and some security guys are a must-have).
To sum up your question. Higher parlours increase your income, but also draw some robbers attention.
