Reasons for rooting:
Free WiFi Tether (sprint, and many other carriers charge for this function)
Ability to remove stock bloatware (most carriers prevent the removal of stock applications)
Ability to overclock, load modded firmware, etc (this requires the installation of a recovery mod which is not covered here).





WARNING
ROOTING YOUR PHONE VOIDS THE WARRANTY AND MAY 'BRICK' YOUR PHONE, OR RENDER IT PERMANENTLY UNUSEABLE, BY FOLLOWING THIS GUIDE YOU AGREE TO NOT HOLD ME RESPONSIBLE FOR ANY DAMAGE TO ANY DEVICE AS A RESULT OF FOLLOWING THIS GUIDE. USE AT YOUR OWN RISK!!!



NOTE: Throughout this tutorial, you may find that the terminal stops responding to your commands (it will just sit on a blank line instead of echoing the command back).  What I had to do was ctrl+c and re run the command (sometimes entering ‘adb shell’ first to return where I was).  

This method still works as of the ~4/20/2012 OTA Update.



Steps to root HTC Evo Design 4G:

Source:
http://forum.xda-developers.com/showthread.php?t=1343114

==PREP==
Download the HTC drivers. ( http://www.mediafire.com/?jfi4bq3udrabw5a )
Download the HTCSupertoolV3.rar ( http://forums.acsyndicate.net/showthread.php?3153-UTIL-WIN-01-04-ACS-HTC-Super-Tool-v3) and extract it to your desktop.
Plug your phone into the pc and enable 'debugging' (menu > Settings > Applications > Developer > enable usb debugging)

Open a command prompt and navigate to the directory where you extracted the files 
EXAMPLE: c:\users\<username>\> cd Desktop\HTCSuperToolV3\root
now you should see the following


c:\users\<username>\Desktop\HTCSuperTool\root\>



==EXPLOITING PHONE==
We will now push the file needed to exploit the system
Enter the following commands into the command prompt

adb push tacoroot.bin /data/local
adb shell (this will change the prompt to a '$')
 chmod 755 /data/local/tacoroot.bin
 /data/local/tacoroot.bin --setup

The phone will now boot to recovery mode, press and hold vol +, vol - and power to reboot.

in the command prompt, type:

adb shell 
  /data/local/tacoroot.bin --root

The phone should now reboot into a reboot loop (as of the latest OTA update, 4/20/12, it did not reboot loop, but I still had root access to push the files) so just bear with it.

You should also return to the "c:\users\<username>\Desktop\HTCSuperToolV3\root" prompt

==THE DIRTY WORK==
At the 'C prompt' type the following commands:
(don't worry, it looks more daunting than it is)

adb push busybox /data/local/tmp/

adb shell
  chmod 755 /data/local/tmp/busybox
  /data/local/tmp/busybox mount -o remount,rw /system
  dd if=/data/local/tmp/busybox of=/system/xbin/busybox
  chown root.shell /system/xbin/busybox
  chmod 04755 /system/xbin/busybox
  /system/xbin/busybox --install -s /system/xbin
  rm -r /data/local/tmp/busybox
  mount -o remount,suid /dev/block/mmcblk0p29 /system
  exit

Now pushing the rest of the files out to the phone.

adb push su /system/bin/su
adb push Superuser.apk /system/app/

Now setting permissions on the rest of the files.

adb shell 
 chown root.shell /system/bin/su
 chmod 06755 /system/bin/su
 rm /system/xbin/su
 ln -s /system/bin/su /system/xbin/su

==FINISHING UP==
Ok, now we have the files required, now it's time to undo the exploit to return the phone to a useable state.

adb shell 
  /data/local/tacoroot.bin --undo


=END==
There, easy as that, your HTC Evo Design 4g is now rooted. open an 'adb shell' when the phone is up and run 'su' to see if you get a # and enjoy!