[The Pareto Principle] guides us to spend little time on many things, without over investing in any one thing...
No. That is precisely what the Pareto Principle does not do. The Principle suggests that we spend all our time on a relative handful of "things", because that will produce most of our desired result. In other words, the Principle is "'overinvest' in a few things, and disregard the rest for the moment".
Why is (say) Apple Computer "perfect"? [1] Because Apple Computer looked at all the things the company might do with its time, then chose to invest in a relative handful of products. The list of things that Apple could do (like: seriously compete with Microsoft in the office-suite market; seriously compete with Linux in the server market; seriously compete with Asus in the cheap netbook market) is far larger than the list of things which Apple actually does.
Moreover, the Principle is fractal. Why is a single Apple product, the iPhone, "perfect"? [2] Because they looked at all the things they could do with a phone product and decided to "over"emphasize a relative handful, like "the web browser". Why is the iPhone's web browser "perfect"? [3] Because they focused intently on certain features, at the expense of others. The browser famously did not support cut and paste on initial release, and it still won't play Flash... but these don't stop it from being a delightful product.
In addition, the Principle is iterative. If you have a goal, you should see if you can get 80% of the way there with 20% of the effort. And, once you've put in that effort and are 80% of the way there, you may find that another 20% of the remaining effort will get you 80% of the rest of the way. And so on. Notice that never does the Principle say that you can't spend 100% of your time working toward your one true goal. The Principle is just trying to help you spend that time wisely.
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[1] They're a very popular example of a "perfect" thing, and this author is not immune to their charms. But, of course, in reality Apple Computer is far from perfect. They have all kinds of problems. But the company is very, very good at figuring out which 20% of the tasks they need to do to convince 80% of their customers that they are perfect 80% of the time.
[2] It isn't, of course. In ways that are really noticeable to, say, 20% of the market: Doesn't have a hardware keyboard option, doesn't allow you to drain your own battery by launching background processes, has a proprietary app store with strangely arbitrary rules, has an exclusive carrier agreement with AT&T and therefore drops a lot of calls. Et cetera. But 80% of the market doesn't notice that.
[3] It isn't, of course. But it's really good at faking it, on a fraction of the effort. Notice a trend in these footnotes yet?