for search engine traffic: usually no
if you want viral traffic and for your site to really take off: yes
think about it:
digg.com (4 characters)
google.com (6)
ebay.com (4)
yahoo.com (5)
baidu.com (5)
amazon.com (6)
myspace.com (7)
etc.
The thing that these domain examples you gave have in common is Millions of Dollars in Advertising and Marketing.
Nobody in their right mind would type in GOOGLE had it not been branded.
Length and generic names are trumped by marketing every time.
TheWomensGolfSchool.com gets a TON of traffic despite its length. Not because women like them long, but because the domain and the business behind it is well marketed and branded.
So the question REALLY is, for "Direct Navigation" is shorter better?
Resoundingly yet for "shotgun" thinking and surfing.
But consider this... Parking pages are getting to be SOOOO prominent on the Internet that even novice surfers know one when they see one. Polls show that an increasing number of users will make a conscious effort to BACK OUT of pages that are obvious parking pages and avoid them like the digital spam mail they really are.
Consider also... short, generics are flat impossible to come by for any SANE amount of money and businesses seeking to establish themselves on the Internet are HAVING to look at longer, more descriptive names, the use of hyphens and alternative extensions to get their "message" across.
Finally consider that regardless of search traffic or direct navigation, the more words used, the more focused the results. If a surfer wants to find information on say... Credit Line Rates... they wont type in "credit" in the search or address bar and muddle through Millions of results. They may type in "Credit Line" or "Line of Credit"... but ultimately, whether it be frustration of not finding anything close to their needs or savvy searching, they eventually start adding more words to their search parameters to narrow the results list... In this example "Credit Line Rates" would give them a the current rates for credit lines versus every Joe out there offering credit cards, lines of credit, home mortgages, refinancing, etc.
As a result of the above considerations, LONGER domains are getting more traffic and use. I just sold DubaiPhoneDirectory.com for a nice sum... what is that? 23 characters in all?
So the answer to your question, Dcristo, has to do with WHAT you are going to DO with the domain you are considering. HOW you are going to market it and WHAT you are going to put there.
If your plan is to do as little as possible and earn as much as possible... yeah, spend $100,000 bucks on a 6 letter generic and sit back and earn some parking revenue and hopefully in about 10 years, you'll break even.
OR...
Build something valueable, regardless of its length.
Just my thoughts... for what they are worth.
GoPC
