I would not say that there is anything wrong with Sea-monkey's calendar. I depends on your needs. Sunbird is a newer, re-worked calendar.
Someone correct me if I'm wrong. But it's my understanding that Sunbird, Firefox, Thunderbird and other Mozilla apps are the bleeding edge of Mozilla's efforts. A proving ground you might say. Eventually the proven stable code makes it into the Sea-monkey suite. Or some may not make it at all if it turns out the users don't like it.
The Sea-monkey suite has a smaller footprint than all the individual apps (Sunbird, Firefox, Thunderbird, etc) because the Sea-monkey apps share share the same base code. The individual apps (Sunbird, Firefox, Thunderbird, etc) can run without the others so you end up with duplicate executables and libs in separate directory's.
Mozilla has no shortage of calendars. Thunderbird also has a calendar extention (Lightning). I've considered building it, but how many calendars do "we" need?
I hope that was helpful?