I use manual exposure and manual focus when using my macro lens. Having a K10D, I would lose an essential function with an AF macro lens.
Snap-in-focus requires the use of a manual lens on any body other than the K20 or K-7. In this mode, all that is needed is to put the camera into AF.S and set the lens focus to the distance (or magnification using the scales on a macro lens) and press the shutter release fully down while keeping the ( ) in the middle of the viewfinder on whatever you want in focus. The camera will take the image when it detects focus. With the K20 and K-7 you can do this using the AF/MF switch on the lens and a menu setting.
Manual Exposure is essential when chasing bugs around the garden or moose in the woods. I set the exposure using sunny-16 or by metering on a suitable subject. I don't want the camera to underexpose (as it wanted to with this image) or overexpose as it would when the subject is in front of dark green foliage or with dark woods in the background.
Here's a sample shot of what I mean - Least Skipper (we think - definitely a skipper, probably a least based on range and the striped antennae) on a pansy. Exposure would vary violently depending on the background. The lovely lady (one inch from nose to tail) was flickering about - that's where the name comes from - and never stands still for more than half a second. Trying to keep it in focus and relying on auto exposure would be extremely difficult.