I believe the cap was put in place to assure that bitcoin maintains value throughout time. If there was no cap, there would be a pseudo-unlimited supply of Bitcoin, which would lead to massive inflation and devaluation. The technical issues with the current blockchain system lasting that long aside, I can't see devaluation becoming an issue. One of the most powerful aspects of bitcoin is that they are made to be divided into incredibly small amounts ( i.e. 1BTC = 1,000,000 Satoshi). Assuming that Bitcoin does survive to each the 21 million cap, that would mean that while there are only 21 million bitcoins, they can be divided into 2.1e+13 Satoshi (which is about 15 times more than the 1.37 trillion US dollars that are currently in circulation). With that many potential units of currency, it's very difficult to imagine deflation becoming an issue.
Hopefully I've somewhat answered your question.