I was asked recently for a few reasons why I still use Expression Media (I still call it iView) rather than depending entirely on Lightroom, so in descending order, here goes:

• By far the biggest reason is to manage in a single place all files related to photographic projects. For me, like very many photographers, that isn't just photos, but might easily include sound clips from wedding shoots, PDF contact sheets, the odd ProShow presentation, as well as any correspondence. Ideally Lightroom should control all these file types, but it doesn't, yet.
• iView's very much faster generating large numbers of JPEGs for emailing or for making a web gallery of a whole shoot. This is because it uses the Lightroom-adjusted preview in the DNG, while Lightroom needlessly reprocesses the raw data.
• I depend on custom fields for recording who's featured in my re-enactment pictures and finding them quickly, and for grouping frames shot for stitching or HDR. While the LR2 SDK does now let you add custom fields, it's too new and undeveloped, and you can't read/write the metadata to/from images. iView does this, so any TIF made from a DNG automatically inherits the original's custom metadata, making it easy to marry up a stitched panorama to the component frames, for example.
• I greatly prefer the flexibility of iView's low tech HTML templates to Lightroom's Lua-based ones.
• iView has scripting using widely-known languages which I can quickly use to copy IPTC location information over to keywords, for example, or to search and replace within captions (eg for typos). It gives me the flexibility to write a simple script in a text editor, or in a well-established development and debugging tool such as Microsoft's VB editors or Apple's Script Editor. Lightroom's comparatively-obscure Lua is an inhuman programmer's language wrapped in layers of nested functions, has little documentation written for non-programmers, gives indecipherable error messages, has no development tool more helpful than a text editor - and in any case has restricted access to metadata. For me, end user access to scripting is one badge which makes a program a professional tool, and it should be present from day 1.
• Using beta versions of Lightroom, and testing them more brutally than may be wise, I want a rock solid DAM base.

There is a huge value in one application combining file management, adjustment, and output. I'm a big user of LR2's smart collections which can automatically group new pictures meeting a wide range of criteria. Migrating to a wholly Flash-based web site, I'm using Lightroom's SlideShowPro export. And maybe the SDK may offer a way forward for my custom metadata. Even though I'm unsure what I want to do about types of files which Lightroom can't import, the balance is certainly shifting and I'm relying more on LR as months go by.