I do not think it makes a difference. Though I can reset the default home page on the server too, I am use to having the default home page as index.whatever, so I change default.htm to index.htm. I have searched EW on how to change the default page setting, but to no avail. (If anyone figures it out, please let me/us know)
In the end, I think it is a) what you are comfortable with and b) the ability to set the default home page on the server.
Otherwise, very few people I know type the page extension when initially going to a web site's home page.
I think that the MS conspiracy theory is a bit much
IIS supports default.htm out-of-the-box. Apache does not support it without extra configuration. For the new WYSIWYG webmaster, finding that their new site "doesn't work" with Apache is as likely to end in using IIS as it is in them first finding the problem and then reading the Apache documentation to find the solution.
It is hard to see this as anything but making IIS more attractive to Expression built sites. Sure, it may be a small thing, but there's a limit to the number of minor annoyances someone's willing to go through just to use Apache.