A debugger for Unity is already on our road-map:
http://blogs.unity3d.com/2009/04/10/unity-roadmap/

It’s probably because they are Unity users already. They know the limitations and Unity meets their project requirement. I know many professional developers who are unwilling to work with an engine that doesn’t have source code. (I’m aware that Unity has source code license but this topic not about the availability of source code license) And I also know many engines that didn’t have source code license initially but later changed it to include. Why do you think that is?
Rune, it’s that you are not attracting many potential users if you have closed policy, meaning, source code license is optional and information not publically available. I say this because from your post, I feel registance or warning about giving the source code license. Even if they do, it might be too expensive for most of users

In my case, I’m giving a try for a simple project that doesn’t require high-end features. But right now the biggest road-block I have is getting UnityWebPlayer to work as ActiveX (http://forum.unity3d.com/viewtopic.php?t=21715). Without the source code, I’ll have to rely on Unity dev to patch it. But so far I haven’t heard anything back from the dev. If Unity dev can be more responsive to user’s request, yes, I can see your point, however, looking at the past release schedules, I’m not totally convinced.
Although Unity is more like an authoring tool, it’s very flexible and powerful but I’m sorry I cannot agree with you on this topic.

BTW, where is the debugger? At Unity, I’m sure you guys use debugger with the complete source code but do you seriously think that “print log” is sufficient to develop a large-size game? Sure you can but at the sacrifice productivity.. Debugger is also essential when you are first learning the tool by stepping though the code…. You can’t even step though your own code but with the source code license, I suppose you can.

Well, that doesn’t seem to be true with all the serious game developers that are using Unity.

> There is no perfect engine that can embrace all types of users. Period. I wish there is a such a thing.

That’s technically true, but that goes for any game engine, no matter if source is provided or not. Having the source doesn’t make an engine magically suited for all needs; it still may take longer to modify any given engine to your needs than to just start from scratch or use a competing engine.

> Debugging is impossible with the blackbox. It’s hard to tell if you err on your side or else.

In reality it’s not a big problem. In the few cases where people have been in doubt, it has typically been sorted out quickly by the community or our support.

> Adding custom features that requires core modification. I already have few list I would like Unity to have. It’s not that you need to modify massively, but simple task may require accessing source code.

“Simple tasks” rarely require accessing the source code. Of course there may be some things you can’t do (ask us before just assuming it, though!) but you have to weight it in against all the advantages that Unity provides.

> If you are targetting casual game developers, it’s probably good enough but if you advocating to professional game developers, you will face the resistences over and over.

I’m happy to report that this does in fact not seem to be the case considering all the high profile clients we are getting!

If you are targetting casual game developers, it’s probably good enough but if you advocating to professional game developers, you will face the resistences over and over.
I heard that Unity is not a middleware from Unity, doesn’t it mean something?

It also didn’t hurt that integrating our conversational AI system with Unity was mostly just dragging and dropping our .NET version into a Unity project’s assets folder.

I also have to say that this comment:

“Users of Photoshop don’t complain that they are limited in what images they can create because they don’t have the Photoshop source code, and the same goes for users of 3ds Max or Maya, or most other creative tools.”

probably hurts the statements you are tying to make as it makes no sense. 2D/3D artists never work with low level source code (they do however sometimes deal with scripting) but programmers do.

Other than that, good article.

i am agree with James and others about comparing artists and programmers and generally it’s not a good idea to compare crazy programmers with any other community.