Money has three functions: unit of account, medium of exchange and store of value. A good currency must do all three well, not just one.

A fixed cap scarcity model is not the ideal scarcity model in my opinion. It's not similar to gold, it's actually stronger than gold's scarcity, gold does not have a cap in its supply, unless you are talking about mass of the earth.

Both Primecoin and PPCoin employ a scarcity model more closely resembling gold. No fixed cap in supply, but scarce nonetheless.

In my opinion, a fixed cap scarcity model is too strong that necessarily sacrifices other functions of the system in the long run, for example, premature weakening of security, or uncompetitive transaction cost.

Things are unveiling fast. Three years from now with another halving bitcoin would no longer be the most secure cryptocurrency against 51% attacks. This is because security is measured by network mining income for pure proof-of-work currencies, and by which measure bitcoin network is losing its share at a fast pace.

http://coinchoose.com/charts.php

Take your pick then, prospects of less security and shrinking market share with a 'comforting' cap hoping for a better value gain, or a more realistic scarcity model with better long term security?