Are there any downside to Windows 8 not doing a "full" shutdown? Would it be good practice to occasionally do a full shutdown to do a full system initialization at boot?
If I'm already booting from SSD is there still a point to using Fast Startup?
A Full Boot differs from a Fast Boot in terms of services and driver loading. Both of them are saved into a hiberfile when shutdown and just resumed when Fast Booted. I can olny see the needing of a Full Boot if you noticed radical changes in drivers(a driver update for example) or the installation of a lot of new Windows Services.
Fortunatelly even when installing new services or new drivers Windows 8 will know about that and will automatically load the new one if it "misses" the hit of a hyberfile open try, so, in a few words, it's kind of subjective, but I don't think it is necessarily to make Full Reboots of your system unless you think you need for some specific reason such as driver update or even due to performance issues.
In addition to what Diogo and Root already said, you also should perform a full shutdown if you have another OS installed or if you plan to boot from an optical disc or a flash drive.
Any change made to the partitions visible to Windows while it is hibernating can result in data corruption. The hybrid shutdown was initially called Logoff + Hibernate, so suppose this still applies. Better safe than sorry...
Even with a SSD booting is not instantaneous. That's because there's CPU work involved to load the kernel, waiting time to detect and initialize hardware, verify configurations, etc. If it was just about the disk, windows would do a full load in about two seconds.
To your question as to whether "it would be good practice to occasionally do a full shutdown", I would like to point that doing a restart always does a full shutdown so unless you never use Restart you may be doing that already.
Also note that disabling Hibernate, a common "tip" for Windows 7 to save SSD disk space, also disables hybrid shutdown/fast boot.
Doing a hybrid shutdown has the following disadvantages:
You cannot use wake on lan feature on some network cards.
You cannot install new Windows updates if you do a hybrid shutdown using the command
shutdown -s -hybrid
Some drivers may only work if you do a full shutdown.
Even if you are using a SSD, hybrid boot will make the boot process way more faster than a cold boot.
Some report that you cannot enter the bios when powering up after a hybrid shutdown.
Read this article for full disclosure on hybrid boot
There is one glaring error in that article where is speaks of using the shutdown switch "full" which no longer exists in W8 RTM
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