You think people ("they") are more competent than they really are. People are idiots, most advances are unintentional, arbitrary, and/or only slightly logical.
How beneficial is it to know old notation?

Algebraic notation lacks the ability to generalize to either flank, or to either player. Some authors such as Pandolfini mix both notations in the same text out of practical necessity, and I do the same. I say you really need to learn both. It doesn't take that long.
http://www.chess.com/forum/view/general/is-there-a-descriptive-algebraic-coverter

Some of the older books are just great great classics, like "500 Master Games of Chess" by Tartakower. You can only get them in descriptive notation. And, of course, much of Chernev is that way too.
I cut my teeth on descriptive and read it fairly fluently, but, given a choice, I'll go with algebraic any day. Reading descriptive now is like being born in Germany, moving to the USA at 15, living here for 25 years, and then having to speak German again. You can do it, but it's a chore.

Algebraic notation lacks the ability to generalize to either flank, or to either player. Some authors such as Pandolfini mix both notations in the same text out of practical necessity, and I do the same. I say you really need to learn both. It doesn't take that long.
http://www.chess.com/forum/view/general/is-there-a-descriptive-algebraic-coverter
I find myself in my room studying the old notation games as if I'm the player of that game and I find myself worrying about everything on the board. I never got that feeling from looking at algebraic notation. It just felt cold and empty, like my blonde friends head. She switched her brain out for an air conditioner.
So, I bought the old version of "The Most Instructive Games of Chess Ever Played." so it is in the old format. I was upset with myself for not looking at the publishing date before buying online. But after calming myself of my 5 second tantrum, I just looked at it, deciphered its meaning and found myself being able to play the games that were written in the book. Then I got to thinking if it is a good thing that I know how to read the old style. I can definitely read a lot more books that I couldn't before, but what if there is something special about this old style that nobody else knows about. What if the new style was brought in by the government to makes us all stupid? What if the government wants to prevent another Bobby Fischer from ever knowng their true potential? What if the modern notation format is all a conspiracy?! THEY'RE BRAINWASHING US! I SWEAR THEY ARE! JUST LOOK AT THE CHARTS! EVERYONE IS STUPID NOW! LOOK AT THE CHARTS, THE BAR GRAPHS, THE CAT VIDEOS! STUPID! STUPID IS ALL AROUND YOU! FLEE! FLEE YOUR COUNTRY AND GO TO ICELAND OR AUSTRALIA! THEY WON'T FIND YOU THERE! FLEEEEEEEEEEEEEEEEE!