Justin and I have it made in this regard -- there's no convincing about the "necessity" of such purchases. The fun comes when we have to decide
who gets to claim the newest purchase as their own.
Over the years, that's resulted in us (trying to) alternate each purchase. For instance, I got the last new camera (my Canon EOS D60), so the next camera (a
Canon EOS 1D Mark II) will be Justin's. Of course, the "selling point" is I'll get his "old" camera (Canon EOS 10D).
It's not quite as easy to justify when one of us wants a new computer, though we've successfully alternated on that as well -- my computer's a little over 2 years old now (we go at least 3 years per PC, with upgrades of memory, video, hard drive, etc. as necessary during that time), and Justin just got a new Dell desktop in January.
We definitely save on film processing and film purchases, and I know I personally take a lot more photos than I ever did as a film photographer. However, we also have invested a lot more into hard drive and backup methodologies (3 DVD-ROM drives in 3 years -- one in a laptop, the other two in our primary desktops) than we ever imagined in the pre-digital days. We went digital very early -- in fact, many of Justin's original prints and slides are scanned digitally on a Nikon film/slide scanner. Compared to trying to sort through negatives and slides, I'll take buying another hard drive on sale or burning more DVDs
any day!
Digital file storage, however, is quite a challenge. You have to get a little brutal with what photos you delete when you realize how many photos you're starting to amass (99+ gigabytes, in our case, and that's not counting thousands that aren't yet in digital format, from our pre-digital days.)