Disadvantages of 32 bit mode I have a Cisco VPN client in OSX 10.6.8, which runs only in 32 bit mode.

Are there any serious disadvantages of switching to 32 bit mode permanently?
Not inherently.  32 bit mode should run fine as long as all your drivers and binaries include a 32-bit version, which is currently the case unless you specifically downloaded 64-bit versions.  It should even use a little less ram.  

But:


Mountain Lion will be 64-bit only, so I don't expect universal binaries to include 32-bit versions forever.
I have "slimmed" all my binaries to delete the 32-bit version.  If you have ever used such space-saving utilities, the slimmed apps won't work.