They team up in the hope of eventually eliminating paper tickets altoghether. Now in a situation where a flight cancels on one of the partners they can rebook you on the other just with an E-Ticket reissue.
Also; I would guess that UA was selected as the first partner for DL because as skymiles partners they already have limited direct access into each others computer reservation systems. That should help things.
My unsubstantiated guess is that the next interline E-Ticket partner for DL would be AA. DL already has a domestic code-share agreement with AA-Eagle at LAX, and those tickets have to be paper currently, and possibly a more expanded domestic codeshare with AA, With UA/US teamed up, why not DL/AA? Again, this is my guess, no information has been given to me about this.
After that I would imagine NW, since DL and NW both use Worldspan, so that shouldn't be too big of a problem. Then CO, since NW and CO have had interline E-Ticketing for years. Next, hopefully they could work something out with their international skyteam partners.