If you want to start trading only with bitcoin and fiat then it will not be constant all the time as sometimes the price come to stability and sometimes a drop down occurs in the price so you have to trade with a number of coins and it is seen normally when one coin start to decrease then any other at that time increase in price so as a trader you will start getting profit from that second coin and with that your profit will be fixed for you.
Trade is indeed to seek profits, but if we do trade in the prolonged period then step is best as you say. i.e. searching or using a lot of coin in order to gain or trade that is done is always stable or that can generate a lot of profit. But if we do trade only aiming for investment, then it would be better to use the bitcoin as the way out, because the price of the bitcoin's getting ahead (in the future) will be higher. This will certainly give you a great advantage for those who wish to trade in the form of investment
well said if we are treating bitcoin as a long term investment yes it will bring us more profits but if you are looking for weekly or monthly earnings having more alts and start playing with it will help you out and earn a lot too, its just needed to have more knowledge about those project then start gaining profits with your knowledge.