Here are NINE very acceptable reasons for the use of an alias.
Use of topical humor when the alias enhances the humor
The creation of a character(literary license) to be used in an informative, fun, innocuously sarcastic, philanthropic, as well as topical way.
Anonymity for when regional or associational sensitivities may be of concern, particularly when there are members out there who will harass you on the boards when you disagree strongly with them.
Innocuous playing of "devils advocate" in a discussion or debate where one wishes to not be further/future aligned with one faction or the other. If you look at the GD Board, there are providers who have flat out stated that they won't see someone based on some of their posts. Why would any guy take the chance.
Avoidance of suffering political fallout for a comment that may have been wrongly construed in a possibly volatile discussion. Some of these regular posters are hyper-sensitive, and also don't read the whole post before going ballistic.
Posting investigative reports, meet & greet announcements, helpful advice, or other similar types of useful information where the use of a user-name could prove problematic or open the poster up to incessant PM's and questions.
Concerns in revealing your review history. This is a big one when you are posting about a particular provider that you are not going to name. In those situations, you open ALL of your reviewed providers to the public guessing who you are talking about. It's not fair to the others.
Ability to contribute where you would otherwise feel embarrassed to speak candidly. Some are just too shy, but will post under an alias.
To avoid a google search turning up your review history. Some of these guys only post in alias for this reason.