A file tree is useful in LR or other DAM progs, for no other reason than it's a handy way of finding files that are not yet added without going through annoying dialogue boxes to import. Just had a play with ACDsee Pro. Very nice implementation of that where you can select multiple folders, drives to import images. EDIT - Not so good after all, as I cannot find how to select a folder and all folders in it to import! You need to browse for your files to place them in your library, so why remove the file browser aspect from a DAM application.
Another really nice feature in ACDSee PRO is the calendar locator, a excellent feature used in Photoshop Elements Organise/Album and I always wished Bridge had. But with added detail, so you can even find images by time of day taken, which I would have never thought of needing, but now I've tried it on some test images it's actually extremely useful indeed. And the more I play with it the more I realise how incredibly useful it is. I remember when I take pics, but not necessarily how I named them, so for me it's very handy.
Also I think there is no way I am going to entrust my overall filing to any Library program like Aperture or LR as they can be replaced by other programmes, have databases become corrupt, they lose images-I just had my external drive change it's drive letter due to another device being added to computer, LR has now lost the [test]images and I cannot simply point the shoot to the new location and reconnect but it seems I now have to reconnect every image manually [if anybody knows of a workaround...]. Apple in particular are quite happy to drop support for older software, so no way will I commit to Aperture.
Whereas if I always carefully file my images by date and a description of folder contents in a logical manner on my HDs, like Diane suggests above, then I can always easily find stuff if my DAM prog goes belly up. It also means when I want to add images to my library I can find them more easily. Not only that, why assume I always want to use LR to look at my pics. Other applications need to be able to access my image files. Say I want to send pics by email or zip them up to email, how would I find them via the OS if I've organized all the images thru my DAM programme. Opening up LR etc to save to images to desktop so I can then find them is a clunky workaround.
A previous bad experience of this has made me very wary of organising programmes. I tried iTunes out a couple of years ago and it decided to completely and pointlessly rearrange my music filing structure on my HD[with no warning whatsoever] ignoring the fact that other programmes may be accessing the files. I only tested it on a very small segment of my music collection, so it only took half a day to sort out!