When the new “MailCo” (as it was at the time) was announced, the first thing they did was poll people on what changes they should be looking to make to Thunderbird as kick-ass as possible – you can see from the initial launch blog post that integrated calendaring was item #1 on a list of new features.
Now, only one year on, the word is that calendaring won’t be integrated or bundled at all – in fact, it gets worse, because the calendar developers have also announced that a number of full-time developers have been lost. Doesn’t say why, but I’m assuming you can blame the Global Downturn™.
I can’t express what a huge mistake I think this is for Thunderbird. One of the main reasons I got involved with Bongo was to provide a calendar server for my day job’s choice of mail client (Tbird), and when Jonny produced his CalDAV code things were really looking pretty sweet.
The problem for me comes down to basically one of maintenance. Extensions that you can install are great, but they have a tendency to bit-rot and suffer versioning issues. Even on Firefox you have issues with a version upgrade making an existing extension wobble or fall over; and with Tbird3 being developed at a more rapid pace, there is a real problem there.
The calendar devs can only concentrate on one thing, and it seems like that one thing has to be a plug-in. In my opinion, a non-working calendar in Tbird3 ought to be a release blocker, but that is now off the table. I have to say, if Evolution on w32 was a viable option, I think that’s where we’d be looking right now, but I’m going to reserve judgement until Tbird3 releases.