It doesnt sound like this is a pre-screening process, if it is, then it may be appropriate, but it sounds more like thsi guy wants to conduct the INTERVIEW via email, which makes me wonder how much time this Boss is going to have to instruct you/train you/listen to concerns you may have/be concerned about employee development and advancement.
Unless this is a VERY fast-paced industry (more than a normal fast-paced industry), or this is a VERY unusually busy time (as in - I just had 12 of 15 staff members quit and they took their employee laptops with them, plus my boss wants a 5 year detailed listing of all the pencils we have purchased on his desk yesterday) I would simply tell the recruiter that you feel that your time would be better spent on opportunities and positions that are truly interested in you. It is so rare (no matter how busy one thinks one is) that someone doesnt have a 1/2 hour to size up a person (the interviewer isnt even spending time traveling for the interview!). Ask what the circumstances are that make him this busy, and if you feel this is an exception to the rule for the boss AND you want this particular position badly enough, then do it. If none of these things apply, set some boundaries - you are worth it!
PS - the recruiter may try to push you into doing it in various ways - dont let him/her. If s/he is truly interested in placing you in the best situation (which generally they aren't, they are just interested in placing you in the best situation for the next XXX time, until they get their commission). A good recruiter will recognize that you have standards, and will look for better listings because of this.
let us know what you decide!