The idea of having a max limit is that inflation is considered bad by most of bitcoin/altcoin developers and users.
With non digital money, you need to add more coins, because the population will increase and you will need more coins to show the wealth difference between those humans. There is also the fact that governments (or those that influence it) somehow like inflation.
With digital money you don't need this. because you can just add a 0 before a number. If the max coin amount is 100 and you need more coins, you just allow people to trade 0.1 coins, and then alow then to trade 0.01 coins and then 0.001 coins and this goes on and on. This is impossible with real money, to add 0.001 dollar coins you need to put them at the market, with digital currency you just allow people to split their "0.01 dollars" into 10 parts of 0.001 dollars.
There is also the fact that you can just ask for transaction fees, or include demurrage where people lost X dollars (or X% of their money) every Y months. This you will have coins to give to miners even after all the coins were mined.