[pdabbadabba]

Whether you want to just rely on a short letter is, I guess, a matter of your appetite for risk. Ideally you would at least provide some argument about why the patent is unenforceable. (In fact, most jurisdictions have rules that require this sort of explanation.) And this would be framed knowing that the other side will have potentially very talented lawyers arguing that it is enforceable.

Since you don't get to file an unlimited number of motions to dismiss on the same topic (for obvious reasons), most litigants find that they want to put a substantial amount of effort into making their case as persuasive as possible before a judge sees it, even if the matter is fairly straightforward. It's the lawyer's job to help the client understand these levels of risk, and the client's job to decide how much money he wants to spend, given those risks.