IMHO any piece of software that makes changes to your system should be view viewed with a certain amount of risk.
We should evaluate weather to use such software upon weather the problem is greater than the risk.
From what I understand about the "Registry" it is no more than a Database which contain entries, over time
and with use some entries become obsolete or "Orphaned" the Processing cycles must be used to "Read" these "Orphaned"
entries therefore taking longer and wasting time, processor cycles, system resources.
Deleting these entries and in theory speeding up your system is the main idea.
From a psychological viewpoint a user might feel like they have accomplished somthing but in reality the benefits are
small.
From the developers viewpoint, I dont think the company would produce and market software if it is going to give them a
bad reputation, I.e screw up a users system. They ultimately want to sell us their products.
People who have had issues in all probability had major issues before they used the software anyway.
One last thing, I think that the amount of Junk software on peoples system is the main cause of probems not
using maintenance software, when ever someone asks me "Can you yave a look at my computer", as soon as I see the bottom right hand corner of the task bar crowded with resource hogging icons and when I open internet explorer and the entire top third of the screen is made up of every Tom, Richard and Harry's "Tool Bar" then I know what software I will be using to clean things up and unistall as much crap as possible.