I strongly believe in Open Gaming whether under one of the Creative Commons license or the Open Gaming License.
Over the course of supporting older editions many choose to rely on the fact you can't copyright ideas (and thus game rules) plus the legal decisions in various places that allow you to say that you are compatible as long as you don't deceive the consumer that you are an official product. (see razor blades and razors)
This is not a good path to take for revived older edition products.
We need to forge ahead and establish our own identity with our own names and get away from using Dungeons & Dragons. Today D&D it just a brand name tacked on to whatever ruleset it's parent company thinks it ought to be on. We need the old rules under new names. Note the plurals as there are multiple older editions.
Just about every old school author I know will gladly share if you ask. There will come a day when you say "Yes" and a few days later you go "What the hell? I didn't give permission for that." I seen this working on open source projects like Orbiter Space Simulator.By using the Open Gaming License you specify what it is you are willing to share. We are a small community that is growing. We get new people all the time. This problem is just going to get worse the larger we grow.
There is an ethical reason as well. Nobody likes it when somebody uses your work without permission or credit. And that exactly what you are doing when using D&D without the Open Gaming License. The law may say it is OK, but you are telling Wizards to stick it. Despite how much of a dofus you think Wizards is, that neither right or fair.
Especially in the light that Wizard did open much of D&D. That for retaining credit and returning the favor by being open ourselves we can use 90% of the D&D rules without royalty or specific approval. That is just damn generous. We should respect that by using the Open Gaming license ourselves.
Then there is the risk of not using the Open Gaming License. Understand that when you see companies like Goodman Games and KenzerCo release older edition products that they had legal advice. I don't care what region of the world you live in, trying to take advantage of gray areas of the law without the advice of a lawyer is downright silly. Remember too that David Kenzer of KenzerCo is a intellectual property lawyer in his day job.
Yes, I realize that for products like adventure the risk I am talking about is minimal. But understand there is a line and without legal advice you won't know when you have crossed. Too often I seen the reasons given for avoiding the OGL boil down to an excuse to put the coveted words "compatible with Dungeons & Dragons" on the cover. The Open Gaming License resolves this issue by spelling out the items that permission have been given for.
Some of you who are published authors are probably annoyed at this point as I questioned the manner in which you released your products. That I brought up ethical reasons that for many are matters of opinion. I think to have published at all for older edition is a marvelous thing. Anybody who asks for my help will get it whether for maps, or ideas. I don't check for an OGL stamp at my door.
My devotion to old school gaming is not blind and I am nobody's cheerleader. I will raise issues that need raising and question the wisdom of actions taken. If this post gets you to think about the issues then I have done my job no matter how you decide.