Most big blogs I read do excerpts and it's the practice I typically follow unless it's a really short post.
Why do you believe is better to display - excerpts or the full blog post? I'm inclined to display the full posts for the reason that it's better for the readers. I beleive that it keeps them on the page longer as it's easier to scroll down the page.
The issue is that the duplicate content will slow down the crawl rate. As result less of your pages may get indexed.
Two ways to show excerpts on your wordpress blog are:
(1) Replace the_content() function on your pages with the_excerpt() which displays the first 55 characters of the post.
(2) Use the <!--more--> tag which will automatically create an excerpt at the point that you place the tag.
I always use excerpts instead of full posts on the main and category pages.
If a theme doesn't have the excerpt feature built in, I found this tutorial on how to do it:
First: Using excerpts (remember that they must be long enough to tell the reader what the article is about, a simple one sentence is not going to cut it) is great for piquing reader curiosity, without annoying them. Once they're interested, they'll click on the title (by the way, if you're using good titles, that might be enough to get a click) and read your post.
Second: This process allows you to fit more posts on the frontpage, which helps anyone who is just browsing trying to see what you have to offer.
Third: Excerpts also increases the number of pageviews and exposure for advertisements.
Fourth: It's better for SEO - plain and simple. :P
I want people to see the post I show the full post. I only use the <more> tag on long posts or heavy pic posts so the load time is not bad.
When it come to duplicate content I feel that it is totally something different then a full page or a part of a page.
Plus it also bugs me when I have a slow connection and I search for something just to find I need to wait for another page to load because someone is worried about duplicate content and show an excerpts. To me I think all they are doing is creating away to get more ads shown.
Let face it. With all of google search knowledge I really think they understand wp blogs and how they are set up. I don't feel they are slapping people left and right for showing a full post.
This poses another question entirely. What is more important the people reading your blog or a search engine?
In Matt Cutts interviewed Eric Enge Google Crawl MC discusses how duplicate content could cause a decrease in the crawl rate:
Matt Cutts: That leads into the topic of duplicate content. Imagine we crawl three pages from a site, and then we discover that the two other pages were duplicates of the third page. We'll drop two out of the three pages and keep only one, and that's why it looks like it has less good content. So we might tend to not crawl quite as much from that site.
If you happen to be host load limited, and you are in the range where we have a finite number of pages that we can fetch because of your web server, then the fact that you had duplicate content and we discarded those pages meant you missed an opportunity to have other pages with good, unique quality content show up in the index.
Eric Enge: That's always been classic advice that we've given people, that the one of the costs of duplicate content is wasted crawl budget.
Matt Cutts: Yes. One idea is that if you have a certain amount of PageRank, we are only willing to crawl so much from that site. But some of those pages might get discarded, which would sort of be a waste. It can also be in the host load realm, where we are unable to fetch so many pages.