Humor me while I think out loud here:
Why we still need these big birding events:
* Not all birders are eBirders...yet! For many birders, the only data they officially contribute to citizen science is by participating in these events. And that's okay as we all enjoy birding in different ways.
* Intensity and thorough coverage of specific regions by hordes of birders at one time. eBirders collectively are gathering fantastic amounts of information and it just gets better with time. Yet, nothing can compare to the intensity of a Christmas Bird Count.
* Social aspects: it's fun to get together with other birders and being part of something bigger than oneself is exciting.
* Birder motivation: the big birding event itself motivates many to get out and look for birds, and to go places, and record sighting data that they otherwise might not do.
* Historical records for comparison and research. eBird hasn't been around as long as many of these big birding events. Hopefully these historical records will be added to eBird to make eBird all the more comprehensive.
When I participate in big birding events, I submit my bird sightings to both the event and to eBird. I hope more birders will do this too. I believe eBird is also working on ways that they can share their data entries with the big birding events so that participants don't have to submit redundant checklists.
So, I conclude that eBird does not diminish the value of the big birding events, but in fact compliments them and vice versa.