Personally, I consider all folders, locking or not locking, non-knives. To me, they're somewhere between scissors and letter openers. The only real knives are fixed blades - certainly they are the most traditional of all knives, and the most versatile in use. How's that for starting trouble?
If I may expand on this a bit. I think that most of us can agree that a fixed blade is by far the superior tool. It's stronger, safer to use, more accessible, more reliable.
Folding knives are a compromise. They make the knife easier to carry (basically cutting the size in half), safer to carry (less chance of stabbing yourself during a fall, etc), more sheeple friendly (I wish this were not an issue, but it is).
Modern folding knives have the best of both. They have the small size, safe carry, and sheeple friendliness of a folder, but they are also
almost as easily accessible as a fixed blade and some of the modern locking systems are
almost as safe.
If you want to argue the merits of the way a slip-joint looks or of their nostalgic/historical value, that's fine.
But I don't think you can argue that modern knives are
functionally superior (whether any of these advantages are
necessary or not).
Maybe you don't NEED to open a knife with one hand, and with proper use, most of the time you shouldn't NEED a locked blade, but let me ask you this....
Can you think of any situation where a slip-joint is a BETTER knife than a modern fixed blade? Is there anything a slip-joint can do that a modern knife can't?