It seems to me that for the most part formal invites are sent out more for sentimental/ traditional/ keepsake reasons moreso than practical reasons when it comes to DWs. Most guests have commited to coming (or not) well before the time that invites go out (6-8 weeks).
The wedding police aren't going to come and drag you away if you don't do them. But personally, I would do them. I can't explain it but there is this giddy happy "this is really happening" feeling that you get when you send them out. Plus relatives/ friends who can't make it can still have a keepsake of your day.
Online Printers like Vista Print can print up invites inexpensively and might not cost you more than DIY invites. Plus it would save you the headache of one more thing to do. We eloped but did our announcements, AHR invites, and thank you cards for around $100. Personally, I'll throw money at a situation to save me headaches.