Honestly if you're not doing SL150 fight clubs there is literally no reason to keep yourself capped at any specific soul level such as 150 unless you just want to give yourself a handicap against opponents. Eventually your soul memory will outstrip your chosen SL and you'll be fighting SL250+ people. And since the Brotherhood of Blood arena doesn't care about Soul Memory OR Soul Level (just what journey you're in, ie NG, NG+, etc) if you go into the BoB arena at SL150 enjoy fighting against people who are SL300 such as myself.
The equivalent analogy is keeping yourself capped at level 25 in cRPG (not spending attributes or skill points beyond what you would have at level 25, of course).