I would absolutely second this. Fade to transparent would be much more useful, personally.

Isn't that what dissolve kind of does? In which scenario would this be useful?

Farid, not to the best of my understanding. Dissolve is a transition, working on two tracks. You can also dissolve to the black track, but that's what the fade from/to black effects do, too. Albeit the dissolve may be a way to work around the current issues of the fade from/to black effects when using as a user effect, for instance. However, you cant store a dissolve transition as a user transition.

...which sparks the idea of user-defined transitions???

One more thought... there isn't technically afaik alpha in video, right? So if you apply this fade to transparent on one track (with nothing under it) then it would fade to black?

Yes and no. Each frame has an alpha channel. But depending on the encoding format there may be no alpha encoded in the source footage. As Kdenlive adds a lowest black track, and with the internally added track transitions, the result would be fafe to blaclk. However, I would not change the existing fade to/from black effects as these do a dissolve with black. With fade to transparency disdolves to other colors or clips would be straightforward to most newcomers.