Biggest disadvantage? would be that all of these altcoins are depending to bitcoin's movements,it means if bitcoin's failed most of these altcoins are going to bleed,causing most of the investors to lose their money which is why it is risky to invest most of your money into altcoin,i personally have 70% bitcoin and 30% altcoins in my portfolio to be safe from these market manipulations.
I know that you are correct when you say that altcoins are very dependent on the price of bitcoin, however to strategy that you have devised in order to avoid the manipulation of the market does not work, if you have your money in bitcoin and altcoins and then bitcoin goes down then what do you think it is going to happen to altcoins? Altcoins are going to go down as well and with it all your portfolio.