Do I think that writing exploits should be a "black art"? No. The full disclosure that HD Moore and the Metasploit team bring to the industry has done a tremendous amount of good by forcing vendors to improve their products, patch when bugs are discovered (and published) and develop secure testing methodologies. These are just some of the improvements to the industry.
A current example is the recent .ANI vulnerability. This exploit came out of work done by Alexander Sotirov. It bypassed all the current protections available such as GS, DEP, ASLR, and IE7's Protected Mode.
This exploit targeted a bug that had already been "fixed and "patched" by Microsoft. Without the efforts of the security researchers out there this exploit would have been used by the usual cadre of spammers, phishers and bot herders and we would have been none the wiser. By releasing this exploit it forced Microsoft to release an out of cycle patch to fix the bug.
So is there a danger that script kiddies will use these tools to "go forth and do bad things"? Sure, but as with anything, you have to assess the risk posed by this and figure what the impact would be to not have access to these tools.
From an interview with HD Moore:
"Some pen-testers prefers doing things "by hands" and don't believe in automatic tools... do you think Metasploit is giving more power to script kiddies, or pros need it as well?
H D Moore: The Metasploit Framework is definitely a "hands-on" tool. Every aspect of exploitation can be controlled, configured, and monitored by the user. Many of the convenience features, such as automatically attaching to a spawned command shell, can be disabled at run time. The automation features in version 3.0 are crude and would likely cause havoc if used on an enterprise network. The Framework is a great way to enhance existing tools and skill sets, but will never replace the role of the penetration tester or skilled analyst. On the flip side, you really need to understand security testing to effectively use the Metasploit Framework. The user must select an exploit, understand which target would be most effective, and choose a payload appropriate for the task. Compared to commercial solutions like Core Impact, Metasploit has a high learning curve and a serious "geek factor". We like it that way. "
If you think Metasploit is easy, you should try Core Impact. It's drag and drop exploiting at it's finest.

If I am doing a pentest does using a 0-day to gain access to a client site have validity? Sure, but once again it's all about risk and impact. Honestly, the client is more concerned about the public exploits. Also, there are far, far more vectors than just exploits to gain access to a site.
Just my $0.02.
-dean-
The full interview:
http://www.securityfocus.com/columnists/439