The original reply was lost during the summer crash. Re-writing it...
You're actually lucky to even have switches to turn them off. Most all newer drives, including my Western Digital and Seagate external drives, now "auto sense" when a computer is on, and automatically turn on and off with the system. This is great if you use them all the time, but terrible if the main purpose is to "backup" the system. Most of mine do need to be on all the time, including the older 400GB WD USB2 drive.
The older 400GB WD drive has a switch, but it will not turn off unless the system is off or the connection is somehow severed. And then I must remember to turn it off manually.
Pulling the USB wires is NEVER a good idea. In Windows XP, there should be a small icon to disconnect external devices.
Right-click on it and select "SAFELY REMOVE HARDWARE" and it should severe the connection to the drive. Then you can turn it off.
Transfer speeds are affected by several factors. One is the CPU, as USB does use some CPU power to control the bus. Another is the quality of the USB2 chipset. In my main desktop system, I use an ADAPTEC 6-port USB2 (PCI) card. Compared to Adaptec, which pioneered a lot of the connectivity formats (SCSI, USB, FireWire, etc), everything else is pretty much inferior. External USB hubs should be avoided, when possible, and you get what you pay for (again, Adaptec is best, although I would recommend D-Link hubs too). My main laptop has hit-or-miss transfer speed on the USB2 ports, and it's the USB2 chipset that does it. The motherboard USB2 slots can be hit-or-miss too.
I leave most of my drives plugged in and active, yes. In fact, I will VPN remotely to my system from my other office, and need access to the files on those USB2 drive.
For "backup" drives, I've started to resort to making my own USB2 drive. I buy the $40 Antec enclosures (black aluminum) and get Seagate 1TB SATA/300 drives. The Antec has a switch that powers on and off. I tell Windows XP to disconnect the link to the drive, and then power it off. I use another to travel between offices.
Hopefully by explaining what I do, I've shed some light into what options you have, for maintaining USB2 drives. If not, ask for more information.