Altcoins are like penny stocks. They can rise significantly or go to zero. So, buy altcoins or not depends on your risk tolerance.
The growth rate of some crypto-currencies for the year 2017 grew by 10-20 times. Among these currencies were: Bitcoin, Litecoin, Ripple, Etherium and others. I am sure that in 2018 there will also be those, whose yield will be higher than the deposit rate in any bank in the world.