If you've read my posts, you know that I'm a pretty strong proponent of no restrictions on talent specs or loot distribution. I feel that such restrictions are inefficient, and in the long term will be detrimental to the raid. One of the major reasons that I think restrictions are bad is that they interfere with developing skill.
I'll start with a definition:
Skill is the ability to make good decisions.
A person who makes better decisions is more skilled than someone who makes poor decisions. In a raid, this can be decisions with positioning, or skills used, or timing of abilities. For a healer it can mean deciding who lives and who dies. But it can also be decisions on how to spec, or what gear to take.
Skill is not absolutely innate. It is learned. As you use your abilities you learn how to make better decisions, and thus you become more skilled.
Restrictions on gear and spec make the decisions for you. Thus you cannot truly improve your skill so long as those restrictions bind you. If you are prevented from speccing Shadow, you will never learn exactly how Shadow limits you in raids, and that knowledge is valuable for future decision-making.
I believe that a Holy/Disc priest who has tried Shadow is a more skilled priest than a Holy/Disc priest who has never gone to the dark side. A skilled paladin will have tried the different options, and makes a decision based on the knowledge gained.
This even extends to loot. If you are prevented from taking loot that you think will help your character, you will never learn how to value loot appropriately. I have an Unstoppable Force, and it is seriously annoying me. It's next to impossible to keep a Judgement stuck on a mob. The lesson learned is that I don't like really slow weapons, and they are ineffective for my playstyle.
Of course, the problem is that sometimes we only learn by making mistakes. And occasionally that will mean spending DKP on the "wrong" loot, or using the "wrong" talents. But if a player is prevented from making mistakes, how can they learn?
Choice is intricately bound to skill. Anything that interferes with a player's choices also interferes with a player's skill development. A guild that is truely interested in having skilled players would be best served by allowing members to make their own choices, understanding that this means that sometimes members will make mistakes.