But what if you write a library or something that becomes popular, very popular or very useful, and your users and others (maybe yourself) want to add a great feature, but it is something that is very difficult to do, or would require more than a single developer to do - in short, it would take a while to do, if possible at all...

...then company B comes along, sees how well you're doing, and says to themselves "you know, this would be great to have that market - and oh, look - the BSD license...perfect!"

So they take your code, add in the changes the users are clamoring for (because they have the resources to do so), then make it all proprietary (perhaps - if you're lucky - with a message buried in 4pt font somewhere that say's a "thank you" and "copyright, etc" to your efforts, as required by the BSD license) - then they market the hell out of it to your users.

Your users see it all - and the flock to it, leaving your efforts to wither and die. You won't be able to catch up, and those extra features they added will keep your old users happy. Perhaps they release an API to allow others to use it, but never the source code.

They've now locked it up, and all your hard work is for nothing. But hey, you used the BSD license, and all's good, right?

/...and this is why OSX is more popular than BSD...