I *always* buy it for the way home. I would hate to forget to check in or have to stop what I am doing on my last day of vacation to try and get internet access so that I can check in and get a reasonable boarding position. I hate to be in the back of the plane so I prefer to board as soon as possible, but that's just me.
For the trip down, I guess it depends on when our flight is scheduled. If it's super early, then I might buy it just so I don't have to get up early to be online at T-24 to check in. Or, if I am going to be at work at the 24-hour mark, then I will definitely buy EBCI because there is no way to know if I will be able to check in at T-24 if I'm on the ambulance or on a call.
If you are OK with not sitting with your travel companion and sitting in a middle seat, then you do not need to worry about buying EBCI. The worst case scenario is a middle seat, away from your travelling companions. If you can handle that, save the money.