I do agree that the main triggering event would be the big problem with this kind of scheme, and that would imply some sort of centralized timekeeper to create the events. I also think that such an event driven block creation system would ultimately give out about the same number of "new" coin blocks as the current system, and it would create much more network bandwidth trying to negotiate "winner". It would also introduce scaling problems that currently don't exist in the current network.
Not to mention we'd also add another two new points of failure - the centralized timekeeper (presumably a network of NTP servers) and the automatic rejection of all valid blocks from clients which don't have the clock set correctly, either because they don't have an NTP service configured or because a firewall is blocking the NTP packets.