I can't find fault with your arguments because I'm sure they're true for many cases (and the 'plugging the holes' is entirely valid).
That said, my gut feeling is that at least a plurality of people are Googling terms like 'basecamp' rather than typing them in as URLs. And I doubt that those who do type in URLs are fickle enough to write off the task on one failure. Proving it either way would be laborious (but would make for interesting research).
I always try to subtly correct that, it irks me when misspelt :P I'm sure people will google for things (hell, I have the "I'm feeling lucky" shortcut bound to "l" on my browser so I do "l basecamp" and I'm there), but this only works if you're first on Google for your name, which isn't something many companies can boast, and certainly not when they're just starting out, which is the time you want new customers the most...
That said, my gut feeling is that at least a plurality of people are Googling terms like 'basecamp' rather than typing them in as URLs. And I doubt that those who do type in URLs are fickle enough to write off the task on one failure. Proving it either way would be laborious (but would make for interesting research).
Good catch on my phased/fazed mismatch, btw! ;-)