iTunes drm files include AAC audio, M4V and M4P video, if you don't remove itunes drm it's impossible to play them on any Apple incompatible devices, like Zune, Creative Zen, PSP, mobile Phone, etc. This turns out really bad if you want to buy a new mp3 player which is not manufactured by Apple.
So, The way not to delete itunes drm files, you could try some methods and programs that remove DRM out of the iTunes files. Please note that using programs might be illegal in the country where you are living, make sure you know what you are doing before downloading and using them.

1.Remove iTunes DRM protection using iTunes + CD-R
Follow these 5 steps:
1.Create an iTunes playlist containing your purchased songs.
2.Insert a blank CD-R or a CD-RW.
3. Burn the playlist to disk.
4.Re-import the songs after a successful burn.
5. Deleting the original purchased tracks.

2. Remove iTunes DRM with DRM software in Windows
One of these program is Daninusoft Media Converter Pro which can remove iTunes DRM with its newest version (2.1.3.0). The reason I list it here is the newest version has modified its two major disadvantages compare with the former one: 1. Be compatible with Vista OS. 2. Supports all iTunes DRM files. And with its other advantages: High conversion speed and friendly-use interface it could be a good choice to remove all DRM files.

http://www.wmatomp3-converter.com/digital-media-converter-pro.html




3. Remove iTunes DRM with Apple software in Leopard
The current iteration of iMovie released with the iLife '08 software bundle is iMovie 7.1. However, in order to remove DRM protection, you will need iMovie HD - previously bundled with iLife '06. Once downloaded and installed, start iMovie HD and 'Create a New Project' titled "DRM-free". Save the project to your Desktop.



In the iMovie window, select the 'Medie' tab located between 'Themes' and 'Editing'. You will notice that the top right portion of the interface now offers a media browser. Select the 'Audi' tab and locate the playlist which contains purchased music, or search by artist / title.



Select the DRM-laden song and drag the file from the media browser into the iMovie timeline where it says "Drag audio here to build your project."
In order to export the song, iMovie requires that a visual image be associated with the audio track - iMovie thinks that it is going to be exporting a movie. Simply drag a blank image (or any other image) to the track above the previously imported audio file. Once iMovie has processed the image, select the 'Share' sub menu item from found inside 'Share' in iMovie's menu bar.



Ensure that 'Expert Settings' is selected in the "Compress movie for:" drop-down menu. Selecting 'Expert Settings' mode will provide the options to export the file as an audio file. Select the 'Share' button.
In the Save as… dialogue, select "Sound to AIFF" in the Export drop-down menu, Choose an appropriate directory for iMovie to export to, title the file, and select 'Save'. Depending on your machine configuration, iMovie will compress and export the movie project as an audio file in 15-40 seconds.

Locate the exported AIFF audio file and drag to your iTunes Library. Right-click on the file and select "Convert Selection to AAC". Once completed, 'Get Info' on the new file, edit meta data, add album art, and trash the originally purchased and AIFF audio files from your iTunes library.