Normal hunts with better XP/h - while often respawn is not fast enough and it may decrease our experience per hour - during rapid respawn weekends time of monsters' spawn is fast enough, so in many places it is possible to make visibly better XP/h. Morover we can choose for example only 3 rooms with the biggest amount of monsters from the whole cave which may help to increase XP/h too.
More hunting grounds avaliable - because of much faster respawn a lot of hunting places may be devided into parts, so more teams may have fun at the same time.
Profit - as you said you can use rapid to make some profit, but I think I may give some examples, since we can make profit in many different ways:
- hunt places good for looting imbu items
- hunt profitable places with way better results
Taming Titanica among Oramond Sunken Lands (Seacrest Grounds) - you can use rapid to trigger it's respawn by killing Abyssal Calamaries
Finally you can get achievements "Askarak Nemesis/Shaburak Nemezis" much faster if on your server appeared Princes!