Unmined bitcoins do not form part of the wealth held by any individual, and therefore are completely worthless. As far as the economy is concerned, it's as if they don't even exist. The fact that they will exist sometime in the future doesn't change the fact that they don't exist now, and therefore shouldn't be counted.
Bitcoin is not a corporation. It doesn't have separate share classes. Nor is it gold. Bitcoin is bitcoin. The 'true' market cap must account for its set dilution, considering how quickly most of the coins will come into being. If we use the corporation analogy, then no (logical) investor would factor the number of shares myopically at 11 million to understand the market's value of one's shares or of the company. If mining was slower and linear over the course of many centuries, there'd be better reasoning for tying the real-world market capitalization to a current and contained window of time that still estimates dilution, similar to precious metal pricing. But it isn't. Bitcoin is bitcoin.
If one is looking to get in and out quickly, sure, it's less important. However, for real long-term value storage, it's important to think of the cap conservatively (with dilution). Ultimately, it's not a huge issue either way. Bitcoin is in its infancy of potential. Bottoming or relative stability is nowhere in sight.