5th Edition is a new edition, and as such much of its supplements aren't out yet. It's self-contained in the sense that you CAN play with just the core rules, but as is often the case the game is richer with supplements so people recommend the previous edition's. The 4th and 5th editions are close enough that most of the content can be converted to 5th with no problem.

To give an analogy, consider D&D 3.5E. At it's basic, it's just three core rulebooks that you can play with and invent whole worlds with. Totally self-sufficient. But suppose you asked for advice on playing warriors when it first came out - people would have referred you to Sword and Fist, not Complete Warrior which hasn't came out yet (even though CW is better than SaF). Likewise, if you were to ask for information on the Forgotten Realms setting when the third edition FR first came out, you would have been referred to AD&D products, even though they are often quite difficult to convert, simply because that is what was available at the time.

In time, as ArM5 develops, references to prior editions would fade.
For what it's worth, so far Atlas seems to maintain a higher standard for the ArM5 supplements than they did for 4E, both in publishing sequence and in content.

>they why do poeple here keep say look in a early ed.

Presumably because there's more material in the four previous editions than in the brand new 5th.

I'd advise you to use only the 5th ed., though, and to only buy previous editions' sourcebooks you really need.

Ars 5 is the current version of the game. When we discuss the game it is best to use the current version of the game as our common ground, our language by which we communicate and understand each other.

However as you say, there are numerous other editions out there with supplemental rules. These editions are cheaper and it's your choice to buy them and use them. However if one wants to maintain common ground, referencing these old books can be problematic causing misunderstandings as we discuss rules.

You keep saying people refer to old books, but I wonder when do people say this?

I know that when I've tried to help one particular person, a person who is using Ars4, I have said that for the sake of consistency we use 'old rules' as the common ground. I do this only because he doesn't have Ars 5.

If a person has the choice of using Ars5 or an older version, I wouldn't recomend an older version to use. As a regular reader of these posts, I can't think of anyone who casually says, you should you Ars 4 or 3 or whatever instead of Ars5.

Material to consider,

Chuck