[susan53]

This is what happens when you try and "force" words into predetermined grammatical classes - they don't always fit properly. In fact, they often don't fit properly.

In the Grammar of Contemporary English Quirk et al say of for that it is "On the gradient between the "pure" co-ordinators and the "pure" subordinators." - ie it doesn't fit perfectly into either category. In fact, they cite and and or as being the only co-ordinators that perfectly follow the rule - ie we have a grammatical class of two items!

It helps if you stop thinking that a word "is" a member of a specific class, but that in a specific sentence it may function in a particular way - a clear example is that hard or fast may function as adjectives or adverbs.

If you look at the rules for co-ordinators and sub-ordinators, you can see that for will sometimes function as one and sometimes as the other. These following are just examples of how, grammatically, it sometimes seems to be one and sometimes the other. I'll use and and or as my examples of co-ordinators and because as my example of a subordinator. An asterisk means that the sentence is incorrect.

Some ways in which for acts as a co-ordinator :

a) co-ordinators always come between two ideas. The order of the ideas can be switched around but the co-ordinator stays put. Subordinators, on the other hand, are attached to one of the ideas and can be moved with it. So :
It was an excellent place for a stroll, because it was seldom crowded
Because it was seldom crowded, it was an excellent place for a stroll.
But :
John came in and David went out.
*And David went out, John came in

Here, for would seem to act in the same way as a co-ordinator - ie like and :
It was an excellent place for a stroll, for it was seldom crowded
*For it was seldom crowded, it was an excellent place for a stroll

b) A co-ordinator can never be preceded by another co-ordinator, whereas a sub-ordinator can have a co-ordinator in front of it :

It was an excellent place for a stroll, because it was seldom crowded and because it was so beautiful
* You can go now, or you can wait till tomorrow, and or you can go on Wednesday

Again, for seems to follow the rules for co-ordinators :
*It was an excellent place for a stroll, for it was seldom crowded and for it was so beautiful.

So, here you can see that although for has the same meaning as because, since and as (subordinators) with their causal meanings, syntactically it's not identical to them.

for as a subordinator :

But neither is it completely identical to co-ordinators. For example, when there is co-ordination, if the subject in the second clause is identical to the subject in the first clause, it can be omitted. So in :
He was concentrating on his work and he didn't see me.
the repetition of he in the second clause is unnecessary :
He was concentrating on his work and didn't see me.
With subordination this is not possible :
He didn't see me because he was concentrating on his work.
*He didn't see me because was concentrating on his work.

Here, for would seem to follow the rule for sub-ordinators :
*It was an excellent place for a stroll, for was seldom crowded.

So, we have some instances of For functioning as a co-ordinator, and another of it functioning as a subordinator. It can also function as a sentence connector :

You'll need to stop and rest. For it's a long walk by anyone's standards.

(If anyone is saying Oh but ... to this, I agree with you. For me it's still a co-ordinator here. But explaining why would make the post too long, and is outside the original question so I'm following Quirk et al and will leave it at that)

and of course as a preposition - I'm looking for John - though I've presumed you were talking about its causal meaning.

All this leaves dictionary writers (rather than grammar writers who should be able to point out the problems) with a dilemma. In a sentence like He left sadly, for he had still so much to say, how do they classify it? You couldn't move the for clause to the beginning so it's not a subordinator, but you can't omit the he so it's not a co-ordinator. In the end they just have to plump for one or the other or hedge. The Collins Cobuild Dictionary calls it a subordinating conjunction, the Longman Dictionary of Contemporary English just calls it a conjunction.

Hope that clarifies a bit..