Oooh, nice topic. I'll just put my archaeologist hat on.

The problem archaeologists tend to have with metal detectors is that of destroying stratigraphy. Metal-detecting laws differ in Ireland, N.Ireland, and England (I've no idea whether Scotland and Wales have the same metal-detecting laws as England). In Ireland you'd be lucky to be allowed a metal detector - it's all licensed. In N. Ireland, I believe it's permissible provided you are not searching near monuments listed in the SMR (Sites and Monuments Record). In England, if you own a piece of land it is apparently your right to do what you want with it, regardless of what's there, unless it's a scheduled monument under state care.

Anyway, a couple of issues:

1. Just because a field/hill/ditch/whatever isn't listed as an archaeological site, it doesn't mean there isn't one there. Perhaps it's still waiting to be discovered. Still, because of legislation, if one hasn't been recorded/identified there, the metal detectorists can come in and search there. An archaeologist excavates and determines the date and context of things from the layers they are in, and those which are in proximity. That way we can build up a idea of how the site was used over time. If someone comes in with a metal detector and digs down to find something, they could be disturbing the stratigraphy of archaeological layers, and could be destroying information in order to get to the piece of metal the machine has found.

2. Metal detectors find metal. Archaeologists deal with all types of artefacts and remains - bone, pottery, wood, stone, etc - much of these could be destroyed by people turning over the soil just to find metal.

3. Certainly there are tensions over unscrupulous metal detectorists, and I have worked on excavations where we have had to scatter nails across the site to deter such people. Yes, there are probably unscrupulous archaeologists out there (in fact, I know there are - although perhaps in a different way from not reporting finds - there are some archaeologists who are ripping off their employees and doing a shoddy job, and that's unacceptable too).

4. Many of the excavations in the UK/Ireland these days are rescue digs, funded through development and only carried out because the site will be ruined in the construction process. The general feeling amongst most archaeologists is that if you don't have to excavate it - don't. Excavation is a destructive process, and once something has been dug it is gone forever with only the records the archaeologists have made as evidence. If we think how much archaeological technique has improved since the start of the discipline, think of all the information that has been lost on the early, poorly-excavated digs (e.g. Knossos - an example of pretty damn bad archaeology). With the hope that archaeological techniques can become more "hands-off" and less destructive, it pays to think that the best thing we can do to retain the maximum amount of information, is to leave well alone.
Don't even start me on Time Team!

(Now, would "GUEST, Digger" happen to be a metal-detecting, singing uncle of mine? If so, thanks for the birthday card!)