Web 2 is an evolution, and kids need to know it and use it. It’s necessary to introduce this evolution in their basic education. On the other hand, kids should learn the basic concepts of studies (language, mathematics, etc); because the technology facilitates basic concepts - for example: correct orthography faults, automatic maths operation, automatic translation, etc. We can compare Web 2 with a calculator. First you need to know the basic concepts of math, and next, the same teacher will explain how to use a calculator to do this in less time.
The internet has become a part of everyone's lives. I think learning web applications should be a part of the classroom studies. In this day and age, it should be taught to the children. It's up to them if they want to use the learning for future endeavors such as web and graphic design.
Fifteen years ago, computers were just starting to become part of everyone's job. These days, there isn't hardly a job out there that doesn't require computer knowledge and use. Even truck drivers use computers. Using web 2.0 in the classroom prepares kids for the real world, and gets them comfortable with the Internet as a tool.
The students should learn the different techniques of model and create applications in internet, as the people use these applications isn’t a part essential of the studies. Although in the present this is the new form of interact in the web, in the future we don’t know the form of interact of the people.
The only way to justify making Web 2.0 an integral part of the classroom is if it actually gives a better, or more effective learning experience. For example, if it helps students increase their grades, or score higher on national tests, yes, you could say it's justified. Other than that I can not really say it's automatically justified.
Children are literally sponges for absorbing information. This is the period of their lives when they best learn concepts and facts. Teaching children how to use online calculators does not teach them how to perform addition and subtraction on their own. Teaching them how to search the Internet for information does not teach them the difference between fact or fiction, how to verify the reliability of information, identify and understand bias, or how to summarize information. Teaching them how to blog does not mean they have learned critical thinking skills, learned how to get to the truth, or communicate ideas effectively to others. Using web applications does not even teach them how to troubleshoot computers, assemble hard drives, generate electricity or upgrade hardware, which are skills that will provide jobs in the web 2.0 economy.