That's the difference between designing a logo and an identity.
Try and get hold of a corporate identity manual to see how it's done.

One of the threads on the subject:
http://typophile.com/node/61096

Mostly on the graphic side, though.

I should've mentioned that it's part of an identity design, rather than just a logo. I've come across a small number of corporate identity manuals, all to varying standards, so I'm fairly confident that I can deliver adequate content and instructions for use. Although that may not still be the case when (if) I move on to designing for larger companies. But thanks for the tip - I should definitely try and get hold of some more.

Thanks too for the link - just what I was after - it seems that most areas were covered in that thread. It's also good to know I've not just gone and asked a really daft question.

One problem with using multiple logo weights is that someone may accidentally use the "small" version in a large size and it looks way too heavy. On vice versa.

Another problem is that it doubles the number of logos to keep track of. Most companies already have multiple versions of their logo -- a CMYK version, an RGB version, a version in PMS flat colors, a black-and-white version, versions with tag lines, versions without tag lines, etc., and if you do them in multiple weights then you've got to have 2 (or more) versions for each.

Not trying to talk you out of it, but just be aware of the downside.

No matter what you do, someone will use the logo incorrectly. There is a 99% chance that that someone is in the IT department and "just needed to fit the logo in that footer". They will use the wrong size logo with the wrong color scheme in a location it should never be located with not enough space around it, per the spec. "The spec? What's that?".

I've seen this poor scenario played out at every large corporation I've been a designer at, from Nationwide to AOL to Fortune 500 financial services companies.

I can't count on 10 hands the number of times I've heard "the logo doesn't work on the web" only to find out the graphic designer gave a CMYK jpg reduced to 72 dpi to some poor unsuspecting .NET developer. I think it happens once a quarter :)

Regardless, you should have a set of logos developed, but absolutely stick to the basics. Create specialized versions when the need arises so you can have the client use exactly what they need for exactly what they are trying to do.