How Thunderbird has gotten this far without integrated calendaring (not just via plugin) I have no idea.
Maybe someone can explain but I just don't get this idea. Why is the current setup so bad? It seems to me that Mozilla's got the best of everything.
Want your calendar integrated with your mail client? Awesome, download the plugin.
Want the same great calendar but don't use Thunderbird? Great, download it here now.
Didn't want a calendar cause you already like yours? That's cool too, our email client doesn't force you to waste resources by loading a calendar you won't use.
I personally use the third option and do all my calendaring from my phone, but I can see that many people do a lot of scheduling via email and the first option works well, or that people just like the calendar but for whatever reason don't like/can't use Thunderbird so option two is better.
Making it mandatory just seems like bloat.