In briefly looking at the website just now, the pitch just doesn't seem all that different from what any patent litigation firm will already do for a client whenever necessary as a part of the defense strategy. Unless the patentholder threatening the suit is clearly blowing smoke (e.g., the client doesn't even come close to infringing the patent), or somebody forgot to pay the maintenance fee on the patent three years ago, or something else short-circuits the need to search for invalidating prior art, it's going to happen anyway.
The company might have employees who are already knowledgeable in the given art area, and so might feel more qualified to search for prior art on their own rather than hiring someone else. If the "hitman" has expertise in the particular art in question, that would probably help him/her more than a general "I'll find prior art, any art, for $500!" pitch (with apologies to Earl Scheib).
Aside: I recall that one of the search firms (Cardinal, maybe) had a job listing on their website for well over a year, looking for an M.E. with automotive turbocharger knowledge. I'm sure there were a lot of M.E.'s applying with the promise that they could learn the field, but the search firm apparently had very specific qualifications in mind.
I don't see where the site says that there is no charge unless "weakening art is located". Even if that is correct, it seems like a bit of a red herring to me -- does the client have to pay when the "hitman" turns over the "weakening" art, or only if the "weakening" art succeeds in court (or on reexamination) in invalidating or damaging the patent?
There may also be secrecy issues; a company might not want to reveal to an outsider that it is worried it may be infringing a particular patent. The website doesn't mention whether the "hitman" is an attorney who would fall under attorney-client privilege rules. If not, the "hitman" might be forced to reveal information during discovery, assuming a client had to explain anything beyond "find prior art against patent N,NNN,NNN" to him.
IMHO, the word "hitman" has extremely negative connotations and sounds frivolous in this context, as if the site owner is not taking the alleged infringer's problems seriously. It's cute, but companies being sued for, potentially, millions of dollars tend not to be heavily into cuteness.
Lastly, the grammar and phrasing on the website seem a little odd. While I recognize that there are many highly-skilled professionals for whom English was not a first language, this may be off-putting, and I would recommend getting the site professionally redesigned; if you look around at law firm websites, even the most bottom-feeding attorney has a website that tries to make him look like a god.