I had the very same question. and went looking for answers. A friend of a friend is a CCIE and cotributes to Cisco Press... He said the CCDA cert and knowledge is valuable, and will help you no matter which line you chose to follow... (ie CCNP, CCSP, CCIP) But is not completely necessary for any of the lines.
The opinion i came to after all the people I spoke to and things I read. It's an OK cert to get, Good knowledge to have. but would be superceeded by any of the professional level certs/knowledge. That said I "might" go for the CCDA, but that is only because my position is currently somewhat volitile, and I may want another cert before I go back on the job market. (getting CCNP is would take too long)
DA & NA are at the same level for Cisco Training. They are both Associate exams which on the career path are at the same level
I don't think that one could say that DA is harder then NA. I see your point that NA does cover more when it comes to some of the topics however the design exams from all accounts are much tricky(this is what i am told from an instructor).
Bare in mind that they are completely different paths and cover different aspects (i.e. Designing a network/implementing a network).
Its like saying "MCP in Managing and Maintaining a Microsoft Windows Server 2003 Environment" and "MCP in Implementing, Managing, and Maintaining a Microsoft Windows Server 2003 Network Infrastructure" are not the same, they are the same level of certification, i.e. an MCP and they are studies in different things so its very hard to quantify one being more difficult to the other.