I know it seems like a good idea, but let's face it - white text on white background seemed like a good idea to a lot of people not so long ago! ;)
Any technique done on a massive scale is likely to either:
a) Cause Google to simply implement a broad filter to prevent the technique skewing results, which will probably have a knock on effect to a lot of people (deservedly or not - Sandbox anyone?).
b) Cause your site to be flagged due to an anomalous increase in backlinks over a short period of time.
Now someone referred to buying PR as advertising, but I'm sorry it's not - advertising very rarely turns round and bites you on the ass.
You generally know where you are with advertising campaigns and although monitoring results may not be quite so tangible as with SEO activities, there are less uncertainties.
The worst that can happen with an ad campaign is that you overstep the boundaries of good taste and offend some people - some bad PR, you lost a bit of money but you can recover.
With SEO related stuff, the worst that can happen is more serious (for most) because you can generally lose more and find it harder to recover.
Paying someone else effectively for a service that they don't provide and have no control over is risky. Period. Speak to some ex Top Pile clients.