>>15429399...They really, really do.
Black Writ + Gold Writ + Silver Writ + tips is 503K, for an entire day of grinding. Black ups enemy levels (and thus EXP) and money, Gold ups money, Silver ups trainer frequency. Mix in Gym Leader/E4/Champion rematches and O-Powers and the Restaurants are blown out of the water.
Especially since you only have to issue the Writs once a day. It really doesn't take much to outclass something that makes you pay every time you use it.