A major benefit in creating a username at Wikia is that all of your contributions can then be attributed to you.
If you register, you also have the option of allowing email contacts from your Wikia friends. To email a friend, go to their user page and click on the email link in the toolbox (left of page). When you register, you should include a valid email address (this is optional, but some wiki journals require it). If you are worried about spam, the email address you provide will not be made public and it will only be used for communications with your Wikia colleagues.
About the .com: Wikia is trying to be self-supporting. This means we all have to watch the Ads by Google along the edge of the screen and we get access to excellent wiki resources and server space. There is no further invasion of your world. If you provide an email address for use by your Wikia collaborators, you will not be spammed by Google or anyone associated with Wikia.
Wikia Create a user account (Free, no risk).