For a long time my opinion was against unlock all. It's not Lemmings like and you need to progress to the final boss level.
But after this thread appeared I thought about it and now I ask myself why Lemmings wasn't unlock all in the first place!
It has to do with a (and maybe even the) main point of the game:
You get stuck! (and sometimes over weeks and months)
It's is a puzzle game, so you need to think about a solution to a puzzle. Here the puzzle is to manipulate the terrain to get from A to B.
If you think about a solution and don't get it right away you are stucked at this point.
If you don't get stucked in a pack (getting all the solution after a few tries) the whole puzzle aspect of this pack falls short.
And remember: What are the levels which you will remember forever?
The ones you got stuck really hard and solved them finally after days, months or even years. And often they are the best ones! Example: The original games!
"No added colors or Lemmings" and "There's madness in the method" are
the levels where nearly everyone got stuck (I got stuck for a year back in the days and there are my favorites).
But there is one point that keeps pushing these type of puzzles down:
The levels from this point onward lay dead!If you get stuck hard you often need to take breaks from this level. What do you do?
Play another rank or take a break from the game.
If you are stuck for a long time you maybe loose interest in the pack even if you played just a part of it, because you cannot access the rest! (which usually has a lot of levels where you won't get stuck)
And that's the point why unlock all is in my opinion the best option.
A big part of the game lays dead and some people might loose interest after some time. With unlock all they can explore the game further on and come back to the level they couldn't solve before. (And they will, because that level will haunt them in their dreams! I know it from myself!)
Now they can see the pack in all it's glory and save the frustraiting part for the end.
But don't leave out the frustraiting part, because as explained above it is often the best one when you have beaten it!
At the end of this post a little quote from me I said to Simon at our meet up as we spoke about this topic (translated):
"Frustraition at it's core, if you are too dumb at this point to get the solution, but that is one of the main point of the game"