Depends on the spec of your Mac Pro. If you have got enough resources to run a decent virtual server and run the apps on the physical box go for it.
As for security, obviously the virtual machines is just a number of files, so someone could take those if they could get access to your physical computer. Otherwise it will just be like running any other computer on your home network.
50/50 is not possible with todays consumer virtualisation software on the current MacPro, even if, it would be absolutely overkill for 'simple home server stuff'.
I have Snow Leopard Server running on my MacPro with the most common services, like Address Book, AFP, Mail, MySQL, SMB, Software Update, VPN and an Oracle Server.
It works like a charm even if the virtual machine itself has just 2 CPUs and 2GB of RAM.
The maximum amount of CPUs you can allocate to a single VM is 4, everything beyond that (6 and 8) is still experimental in VMWare 3.0.
But as I said, even 4 CPU cores are absolutely overkill for home services. I never saw the 2 cores I allocated to the Server rising higher than 20%.
So I recommend you try it with the smallest settings you can get and see how it's working. You can always raise them without needing to reinstall the VM.
Nevertheless my Server licence is running out this month, so it's time to get the Mac Mini Server, which should work better for me, anyway.
Running the Pro 24/7 is a little expensive.