If they didn't ask you to forward, would you freely respond with what you in fact are looking for, or would the recruiter specifically have to ask that? I usually phrase my intros by saying if the opportunity I presented isn't something you are interested in, I'd like to learn about what types of things would interest you so I can let you know about only those opportunities if they happen to come across my desk. Response varies, even with that added line.
The forward request is a little grating, but I used to reply to recruiters asking how they came across me, regardless of whether they included that line. Virtually none of them replied back. Lately, I've just been junking any recruiter email without replying back if it's clear they don't know anything, or they know the bare minimum, about me.
If a recruiter emailed me, opened the email up with an explanation of how they came across me and why they thought I'd be interested, I'd be much more inclined to respond with more information about what I'm looking for even if the offer in their initial email is not even close or if they asked me to forward the email to someone else.
A better analogy might the pretty girl walking home from high school, of the rather hick-ish / paleosuburban sort -- when the older dropout pulls up in his muscle car and asks "Hey babe, wanna party?"
I get about the same feeling when talking to recruiters, most of the time.
Go in to a McDonald's and start asking the staff if they want to come work at Taco Bell or Burger King. Interrupt them at home during their time off a few times. "Hey, come work for another company doing essentially the same stuff for essentially the same money - maybe 3% more!" I doubt you'll get many takers.
Which they then package with over-the-top smarminess and intellectual dishonesty. And for this, they add a 30-40 percent premium to your acquisition cost for any potential employer (and concomittant disincentive to hire you, or respectively, increased incentive to relentlessly grill you, check you over for ticks and fleas, etc to make sure you're worth the overhead, if they do choose to bite).
I've worked with a few good recruiters, but even over a 5-month job search and talking to every major recruiting firm in town, I got zero actual job offers (and only one real promising lead that fizzled out on the company's side). There's a LOT of inefficiency in the system.
Contingency recruiting and the incentives inherent in that model, again, is at the heart of the problem.