One of the problems with people carrying out checks on your details is that is also registered as a check on your account, subsequently if you actually get turned down for credit purchase or loan etc, part of the reason can be the amount of checks that have taken place on your details.
If there are too many checks, this then starts alarm bells ringing as to why so many checks, resulting in a refusal of what you wanted on the basis of "better to be safe than sorry" (What constitutes "Too Many" is any ones guess).
The only exception to this, is if and when a check is instigated by your-self as this does not count.