Its like saying you should only buy gold or silver, but not diamonds, rubies, emeralds, or oil futures or apple stock. they're currencies and will be worth whatever the market provides.
what about 42coin that is worth what 100x BTC?
The thing about the gold, silver, gems and stocks is that they are are relative to the dollar, in the same way that altcoins price directly correlate with Bitcoins price. Because altcoins are exchanged for Bitcoin rather than actual physical currency, the price of altcoins will always be dependent on Bitcoin to succeed.
Of course, the amount of a Bitcoin each altcoin is worth is dependent on other factors, such as popularity and acceptance. The reason 42coin is worthless to mine is that it has neither of those two - sure it's worth a lot, but its far more profitable to mine something such as Dogecoin.