What purpose does setting a sticky bit on a directory without the executable bit serve?
drwx-wx--T 2 root crontab 4096 Apr 24 15:00 /var/spool/cron/crontabs/
What you're looking at is similar on Debian. The directory does have the executable bit set, for the owner and the group. Just for the owner, sticky doesn't make much sense, since that's by definition only one user (and the owner gets to override sticky anyway). But for the group, it matters exactly as much as for world-writable directories like
/tmp, namely that regular users can't remove files belonging to other users.
But why would anyone be a member of the group
To be able to modify their crontabs, of course! Debian's
with setgid privilege, thus allowing any regular user to access that directory, with their own UID, and with the GID of
crontab. That's slightly safer than letting them run
crontab with setuid privilege since keeps the users separated from each other.
-rwxr-sr-x 1 root crontab 36008 Jun 11 2015 /usr/bin/crontab
Now, normally, the files in the directory are owned and named by their respective owners, and if
crontab only allows to remove the user's own crontab, there shouldn't be a problem. Having the file permissions set up like that works to protect from bugs in the program, and makes the actual UID of the accessing user relevant, not just their name.