This is not specific to programmers and it is imaginable that a slow learning programmer with domain specific knowledge has value.

While fast learning per se is an asset in any field, however, in many occupations learning seems to be taken as a necessary evil that merely makes you wish you could do things like you used to and go on like that. You get trained to be something and when your field changes, you're forced to learn something new and you only do it then.

Programmers on the other hand—well, their job is to learn new stuff. Sometimes it's a new codebase, sometimes it's a new framework or library or language or protocol or a set of tools but there's always something new. The ultimate new thing is writing a program that hasn't existed before: everything is new. And even that new stuff doesn't have value per se, it's just a prerequisite for a programmer to "get things done". Stuff gets into the fashion and stuff becomes obsolete, but the programmer is riding on the front wave.

Not only good programmers have to be able to learn, they have to be able to learn fast. Programmers can't afford to take a six-month course to learn something new. If you can't learn new stuff and successfully map out the diffs of how it relates to stuff you already know, you're out soon.

As someone pointed out, while being able to learn also means that the programmer has undoubtedly learned a lot, I still argue that the value is indeed in the ability itself. A junior guy with the good programmer aptitude and ability to learn can bump himself into shape in a few years of time. Another programmer who has already learned a lot but who has lost the willingness to learn anything new is toast sooner than that.

Learning is implicit in the job of a programmer. When your job is to circle around the boundaries of unknown, it is something that you can't avoid.