Registry cleaners (e.g., System Mechanic, Advanced SystemCare, etc.) claim they optimize your system but lead to Windows Update errors like 800700641

Noel, that is an excellent illustration that a problem exists, but it needs more specifics.   Were you able to identify specific registers that were flagged by the Registry Cleaners whose removal clearly led to the problem with specific WGA services? 

A solid way prove the recklessness of using registry cleaners is to reproduce the problem, then make it go away by "undoing" the cleaner's deletion.[1]  For example, back up the registers, then reproduce the problem by allowing the RC to selectively remove specific documented groups of RC-flagged registers.  Next, show that restoring the same flagged registers makes the problem go away.   You have now acquired irrefutable "smoking gun" facts.

You could then publish these "smoking-gun" examples that demonstrate the damage of a specific RC's actions.  In fact, you might have a great article that shows how several different RC's do this kind of damage.   I think this would be an excellent way to drive home the problem of registry cleaners.  It would also turn the screws on the vendor to fix the problem.

Again, this would scientifically separate the facts from the hearsay warnings.

1.  Roebear's excellent link (above) does this in an indirect way, but it needs to demonstrate Lolo's before-and-after damage by using it to show it clearly flagged and damaged these specific registers, which can be restored to make the problem go away.   http://answers.microsoft.com/thread/f6fd8e12-e83a-457b-af0a-ae54a09b71b5

they optimize your system by cleaning up your registry.  If this leads to Windows Update errors like 800700641, are these cleaners really worth the trouble?  Advanced SystemCare's "Deep Registry Fix" found over 500 "high risk" registry entries to repair on my PC.  Now, I never claimed to be the smartest guy in the room, but I'm not that dumb; so, I didn't have Advanced SystemCare "repair" these registry issues.  My question is: Why would Advanced SystemCare, by most accounts a top of the line System Utility for registry repair and system optimization, lead their customers into such terrible traps (traps I'm certain the average guy on the street could never recover from on his own)? 

http://www.techrepublic.com/blog/security/why-your-registry-doesnt-need-cleaning/370

The disasters that can follow:

http://aumha.net/viewtopic.php?t=28099

I personally do not recommend anyone use Advanced SystemCare.

...My question is: Why would Advanced SystemCare, by most accounts a top of the line System Utility for registry repair and system optimization, lead their customers into such terrible traps...? 

One word: Money!

More specifically in Mainland China-based IObit's (Advanced SystemCare & System Mechanic) case, American money!

Speaking of registry cleaners (e.g., System Mechanic) & 0x800700641 or 641 errors, see http://answers.microsoft.com/thread/f6fd8e12-e83a-457b-af0a-ae54a09b71b5

...because they can always blame someone else for the problems they create, and it's a good money-spinner.

They can 'prove' how much work they've done - after all, removing 500 entries must be a good thing (ignore the fact that Windows becomes non-genuine, Updates won't work, and if you're really unlucky,Windows won't boot at all!)

Money for old rope - and  - there's one born every minute - are the homlilies that come to mind.