You don't need to keep in mind what is lost, you need to compare what is lost with what is gained. Big difference.

Yes or no depending on what the calculations say. In the end, what people 'feel' is irrelevant, but calculating it is difficult, because some talent builds are more effective in specific combat situation (e.g. single target versus multi-target).

Overall, I'm confident that classes are more or less balanced DPS-wise. Where you lose DPS from losing an elemental, you gain DPS from another skill, perhaps even in another form. (or the same form, cause mages can have a pet too).

When levelling a new group, I chose skills and abilities that I need and then start forming a composition based on what each character adds.
1. The fact that I was espousing a marginal benefit analysis includes the notion that costs and benefits are weighed. That particular statement was a reminder that a 4th shaman isn't just a redundant healer that offers nothing you don't already have.

2. What people value is a key factor in how they behave. It isn't a matter of how they "feel", but where their individual playstyle, goals, and target activities are. If there were only a single, absolute answer, there would be no variation in behavior.

3. Different skills offer different utility based on situation. You may see a larger loss in DPS because the "form" some other skill took doesn't apply as much as the skill of a different class.