That is a reasonable way of thinking of it. That's a really good point among many being raised here. So let's figure that one out.
I discussed the issue of nofollow distorting the web graph with a Googler and I was told that only about 1% of all links are nofollow. Here's a video by Matt Cutts [youtube.com] where he discusses this issue and confirms that nofollowed links comprise a miniscule portion of the web and poses literally an insignificant influence on the algorithm. The important part of this video is his confirmation that nofollowed links comprise an insignificantly small portion of the web.
So in terms of natural link profile, if the vast majority of links are dofollow, does a site with a 50% nofollow profile appear normal? No. It is abnormal. None of my sites, or those I compete with, have such a link profile.
I can understand if in the niches you compete in that a 50% nofollow backlink profile is normal. However, a niche where many backlink profiles contain unnatural backlink profiles does not represent natural linking patterns, it represents webmasters pursuing similar backlink acquisition strategies, like from blog comments.
We know how this works. People observe what the competition is doing (copying the backlink strategies of the leading sites) and join in the strategy under the assumption that that is what it takes to compete. That's why we see entire niches like Real Estate that are utterly infested with old school reciprocal link patterns. It's a herd thing, not a natural linking pattern. Or in finance niches you typically find a huge amount of nofollowed comment backlinks. That's not natural either, but it's what everyone else is doing so everyone follows. It's just backlink research gone wrong.
So here is what we know:
1. Nofollow links comprise an insignificant minority of links.
2. Thus, nofollow links comprise a minority of backlinks to most websites.
3. If the average backlink profile comprises a small amount of nofollowed links, a site with a high percentage of nofollowed backlinks appears unnatural.