In a 100% white hat campaign that you’re confident about, buying links is never recommended as Google can pick up on this and penalise the site accordingly. Most agencies do buy links though, but they are aware it’s strictly against Google’s rules so instead of paying with cash they usually offer something and don’t explicitly ask for a dofollow link even though that’s their goal. They play the numbers game knowing that if they give free products to bloggers to try and review they will usually place a good link in their review page. Some simply offer to write for the blog, and slip links in using their own editorial discretion, pretending it’s a useful part of the article.
Ultimately though, if you were offered a link from adobe.com’s homepage, with all it’s link equity, and it would cost $1m for one year, and you were confident enough about SEO to know that link would make your site popular enough to earn many millions in the first year, it’s really a no-brainer, so long as it’s confidential and not pointing to a domain that needs protecting at all costs. So long as link equity has a value, there will be a market for dealing with it, and people who can earn good profit from it.
Owned media is so much more reliable than ‘rented’ media though, which is what we’d be doing by paying for a link on someone else’s site. Thus acquiring that site, and linking to your own other site, is a better strategy if the numbers add up provided you keep both sites independently registered, hosted, link profiled, etc.