The Cost Issue
The issue at the core of future-proofing is cost. Those who promote future proofing do so in the belief that the cost of upgrading will, over time, be more than the cost of buying or building an expensive but powerful PC. Those against future-proofing believe the costs of upgrades is minimal, and does not compare to the price premium attached to cutting-edge PC components.
With the pace of development and increasing quality of low cost parts, most choose to lean in the direction of not future-proofing a PC. The reason, ultimately, is this; PCs include numerous non-core components that do not need to be frequently upgraded. This includes the enclosure, the power supply, and the hard drives. Future-proofing a computer often means having to invest heavily into these areas in order to support components like processors, motherboards, and video cards. For example, a future-proofed PC built today would likely need a large video card, such as a Radeon 4870 X2. This card will not fit well in smaller, less expensive cases, which means the future-proof computer has to incur a cost that those who do not attempt to future-proof their computer will not have to deal with. Power supplies fall under the exact same rules.
It should be noted that this comparison is becoming less significant as the prices of PC components and pre-manufactured PCs fall across all market segments. A few years ago, a person attempting to future proof a PC would likely have bought an 8800GTX video card which cost around $600 dollars. Today, the fastest video card solutions can be purchased for between $200 and $400 dollars.
That was Unexpected
The more pressing issue against future-proofing, however, is the difficulty of for-seeing where the PC hardware industry is going at any given time. For example, it is impossible to know with certainty when AMD might change sockets. It is also impossible to say with certainty that Nvidia won't abandon SLI in the next few years. Educated guesses can be made, but hardware advances can be unpredictable.
This problem is complicated by future-proofing's reliance on technology which may not be proven. For example, those trying to build a future-proofed PC in the last half of a year likely went with Nehalem. But it is unclear how well those first Core i7 products will hold up against the release of the new models that will be debuting less than a year later. Solid State Drives are another example. Future-proof computers would likely include a Solid State Drive. But what if Intel releases a new storage controller which gives their new Solid State Drives a huge performance increase over their old ones?
Of course, these are what-if scenarios. The past unpredictability of PC hardware does not mean that the industry will make wild moves in the future, though the trend shows no signs of stopping. But it is often better to be safe than sorry.
Final Words
Overall, future-proofing is probably not a worthwhile endeavour. The cost of buying cutting edge components is always extremely high compared to purchasing something which is merely adequete. But if the arguments above did not seem to be absolute evidence against it, there are reasons for that.
There is no refuting the simplicity inherent in future-proofing. Building a future-proof computer that will last the next five years means not having to deal with upgrades for five years. In addition, there are certain components which can last for an extremely long time. The case is a great example. A high-quality PC case might seem expensive, but such a case might be usable for ten years. Ugrading is not always an easy process, and negating the hassle can be worthwhile. People building their own PCs, or with access to these services, can mix and match future-proof and easy to upgrade parts, putting money into long term (the case) and difficult to upgrade (the motherboard) components, while saving on cards, chips, memory, drives and parts that are easier to pop in and out.
Speaking from a sensible, budget oriented perspective, future-proofing is unwise. But it is not so unwise that doing so would be foolish, and a compromised approach that future-proofs some parts of a build is attractive. Anyone who does not want to have the hassel of upgrading or simply prefers to buy the fastest components possible, either for top performance or bragging rights, wll likely find the cost of a future-proofed PC worthwhile.