If you manage to get your website to the top of Google and it's filled with spam links and a poor user experience, what have you accomplished? Nothing, really. There's some money in it if you're one of those spam blog filled only with Google adwords suckering in folks who are new to the web, but this doesn't seem to be a very solid or sustainable way to make money. If you're doing your best to go behind Google's back, eventually they're going to figure out a way to dock your website to the bottom of the rankings. That's kind of their thing, y'know?
But that's not my point. My point is: if you get to the top of the rankings but your website is poorly optimized to do it's job (i.e., sell your products and/or services) then you've wasted your time. SEO isn't an ends, it's a means.
Besides, good SEO will actually improve your website's conversion and user experience. Good SEO means that you use a site map that conforms to what search engines and what people expect. If you build your site according to standards, people will know how to use it. If you make up your own insane navigation scheme, the search engines won't know what to do, and neither will your users.
If you help out the search engines by deep linking to various back pages on the website, you'll also help out your users. Search engines don't want to "click" more than a few times to get to the "meat" of your website, and guess what? Neither do your users.
Good headers tell the search engines what your page is about. It also tells your users what your site is about. Same with page titles, links, and every other aspect of navigation.
Don't suck up to the search engines at the cost of user experience, because in the end you'll be hurting both causes. For example, if we spammed our own website, MarketingBreakthroughs.com, with only one or two keywords, the entire site wouldn't make any darn sense! But we still keep our keywords in mind and make sure they show up in the text. We put them into page headers where appropriate. We try to describe out inbound links as accurately as possible.
It's not simply about balance, because serving one end often serves the other.