One other disadvantage of E-tickets:

Twice, when trying to apply an unused E-ticket to a new reservation, with additional funds due, UA phone res charged the entire fee to my credit card, instead of applying the value of the unused ticket and charging only the difference (plus change fee). (This is much less likely to happen if you're changing an existing reservation that has been E-ticketed, since extra steps are involved in transferring an unused E-ticket to a new reservation)

Once, when making several exchanges of E-tickets for different trips at a UA res office, the agent goofed and accidentally applied the value of a ticket she had just issued, instead of the ticket I was trying to reuse. When I got to the airport for my flight, and tried to use the E-ticket for travel, they told me the ticket was no longer electronic - it had been printed out. It took a lot of persistence to get to the bottom of the problem....if the agent had been in a "brush-off" mood I'd have been out of luck.

The bottom line is - paper tickets are a lot safer if you're a frequent exchanger, as I am (love those United Shuttle tickets with no change fee .