Why does Windows always mount a USB drive as the next drive letter after the last physical drive, even when that letter is already taken by a mapped drive, and is there any way to improve this behaviour?

What happens is I tend to use a few different flash drives on my PC, as well as having both a Blackberry and a personal phone that mount as USB drives when I plug them in to charge. Being on a corporate PC I also have a number of mapped network drives (some set by login script, some set as persistent mappings in my profile).

When I first login I'll have drive letters like this:

C: - Local Drive
D: - DVD Drive
G: - Login script mapped drive
J: - Login script mapped drive

When I plug the Blackberry in it'll mount two drives (one for onboard storage, one for the SD card) as E: and F:. If I then plug in another USB drive it will mount as G:, even though that's already taken by a network mapped drive. This leaves me with the following drives:

C: - Local Drive
D: - DVD Drive
E: - USB drive (Blackberry)
F: - USB drive (Blackberry)
G: - Login script mapped drive
[G: - USB drive - mounted but not visible in Explorer or command prompt]
J: - Login script mapped drive

I then have to go into Disk Management, find the new USB drive that's mounted to G: and re-assign it to another letter eg Z:, once this is done Auto-Play detects it and throws up its normal dialog, and its browseable in Explorer.

While this is OK to do if you only use one or two USB drives and have admin access to your PC with your login account, its a total pain in the proverbial if you regularly use a whole load of different USB devices, and corporate policy means you have one account for your normal login (that only has User access to workstations), but have to use a different account for any privileged action.

I realize that one possible reason for this is the difference between hardware which is mounted and assigned drive letters at the systen level, and mapped drives which are done at the user level. For USB devices that are already plugged in before login, then obviously they're mounted before Windows knows what network drives may be mapped. However if you plug the USB devices in after you're fully logged in and have drives mapped then Windows must know which letters are available?