I mean for the reader. Especially the casual reader who is not already deeply into web fiction.
(And though I'm saying "I" here, but this is Marketing 101. You'll find it in any book on web design or marketing or even the wider subject of industrial design. Never make the customer work to use your product... unless what you are selling is exclusivity -- That is, using your product is a status symbol.)
For instance, when I go to a new serial website, I often find that I have to hunt hard to find links or information. Sometimes there is no way to get to the first episode in the series, or it's left to the blog's automated archive listings, which I have to then click through to find the beginning.
If I have to hunt for anything before I'm hooked, or even if I just can't tell at a glance which link to click on because they have excessively clever titles or images... I'm gone. I get frustrated if there isn't a previous or next episode button. An I really hate it when the web design tries to make me use the site in some way I don't want to use it.
Really good navigation is always a work in progress. You will never please every person. You'll make mistakes too. But generally, the easier you make it for the reader to do whatever he or she wants to do, the more likely that person will stick around and like your site.
Now, two caveats hers:
1.) this is a matter for NEW customers, and also readers who may not be as familiar with serials and therefore don't have a strategy for dealing with a website like that.
2.) I'm not saying that this is anything but one of many issues that MCA should take into consideration. It's a major one for me, because I'm interested in converting a non-serials audience into serial readers. And for that group, there is no harm at all in leaving the serial up while selling the book, because it's just more opportunity for them to get hooked, while always making it easy for them to buy.
For the hard-core serials audience, I would defer to the judgement of anyone who has been at this for a while. Those already succeeding in the niche know best what the expectations of their audience is.