When bitcoin becomes too dominant the altcoin market falls which is bad overall as good alts like ethereum, eos, will also crash. 50% is just about right, if it gets lower then it's use as digital gold might waiver
I also think that too high BTC dominance or too low BTC dominance, both are not good for the crypto currency. Bitcoin dominance should remain between 45% to 55% as this is ideal percentage for the market to grow.