I don't think there's any right or wrong answer to this question but I would like to know how strongly the community feels about the issue.
As far as I can tell, adding ".html" to a page suffix achieves nothing "extra" as far as how a site URL appears. From my experience, looking around the internet, most professionally-run websites do not include page suffixes (e.g. .htm, .html, .jsp, .php, .asp, etc.). That's just the evidence that I have observed.
Joomla has the following option:
» SEO Settings
» Add suffixes to URLs
I do not know whether this option is fully supported in Joomla. I do not know what issues this creates if people use that option (or if people do not use that option). We know that there's a relationship between using this option and an effect has on "message preview": see the topic
K 2.0.2+3: problems with preview
I do not know to what extent that Joomla supports
this option; however, I would like to know how many people use
the option. I have created this poll to gauge public opinion in attempting to quantify the extent to which site owners depend on feature in their webcraft. The poll question is simple. Do you use the feature, yes or no.
People who use the setting can give us their reasons why they think it's important.
My personal opinion is that this setting is unnecessary. Joomla routing takes care of the things well enough without needing to add .html to all your pages, anyway. But maybe there are other reasons that I am unaware of.