Mounting a filesystem in RAM is not just done for speed. The most common case of this is with the average /tmp directory, which is mounted via tmpfs on almost all modern Linux distributions
In fact, instead of mounting /var in RAM, you should consider symlinking the appropriate subdirectories to /tmp
This is what many embedded Linux distributions do (OpenWRT, etc.). Although, keep in mind they are meant to run a much 'smaller' hardware:
1: In the case of logging, are these logs then available to view? - Say the machine crashes or whatever - can you get back to the logs to see what went wrong.
Of course, you will lose these logs on restart. If you're concerned about this, you could consider a remote syslogd
2: In the specific case of the RPi with fairly limited RAM available, is doing this a good idea? - if there's a risk of filling a disk is there not also a problem with running out of memory?
The amount of logs that will get generated is not as large as you may think. It would be easy to keep them in memory granted the machine isn't running for years. (And then you'd simply need to clean them as you would anyway). That said, this also means the amount of data recorded to the sdcard isn't really that significant either.