I tested all the Cakewalk synths in 64-bit Sonar 8.5.3 and found that several are 32-bit only (and use Bit Bridge) and a bunch more don't start at all. 

64-bit:
  - Cyclone
  - Dimension Pro
  - PSYN II
  - Rapture LE
  - Roland GrooveSynth
  - Session Drummer 3
  - SFZ
  - SI-Bass Guitar
  - SI-Drum Kit
  - SI-Electric Piano
  - SI-String Section
  - TruePianos
  - TTS-1
  - z3ta+

32-bit only (uses Bit Bridge):
  - Beatscape
  - Cakewalk Sound System
  - DropZone
  - RXP
  - Square I

Broken ("failed to insert soft synth"):
  - Dimension LE
  - DS864 DXi
  - nPulse
  - Pentagon I
  - VELOCITY

Is this list consistent with what everyone else has noticed?

Are there any workarounds for the Broken synths?  Am I missing much if I cannot get them to work?

I never use Beatscape, Sound System, Drop Zone or RXP so cant confirm if they are 32 or 64 bit.
 
The only "broken" one I use is Pentagon 1 and it works fine for me in 64 bit, It's one of my go-tos, but I have seen other post problems. Pentagon 1 is a DXi. I suspect the problem either arises when only the 32 bit DXi is installed or if someone has assumed it to be a VSTi and has moved the DLL around.
 
32 bit DXi will not work in 64 bit. I suspect this is the reason some are "broken". You did not mention Dreamstation, that is one that definitely will not work in 64 bit.

Are the broken (32-bit DXi only) plugins generally redundant with the other soft synths?  Am I missing anything by continuing to use 64-bit?  Or should I consider switching to 32-bit in order to try out the broken synths?

Pentagon I does work in 64-bit. There is something you have to do to get it to work and I don't remember what it was, but it was simple. Search the forum. 

Tripecac


Are the broken (32-bit DXi only) plugins generally redundant with the other soft synths?  Am I missing anything by continuing to use 64-bit?  Or should I consider switching to 32-bit in order to try out the broken synths?

 
In my opinion you won't be missing much. You can install both the 32 and 64 bit versions of Sonar on the same computer so it would not cost much time and effort if you wanted to try them out.
 
Dimension LE  - 100% Redundant as you already have the PRO version working in 64 bit.
DS864 DXi  - I think this is Dreamstation? If so you are not missing much. It was a good synth in its day but with Z3ta, Rapture, Pentagon, Square etc it does not bring much to the party. You will also find quite a few free VSTi that cover similar ground.
nPulse - Not in the X1 producer package. I really don't know much about this one. Its a drum synth apparenty. I must have tried it in an earlier version of Sonar and not been impressed as I had forgotten it even existed.
Pentagon I - It works, it should be a native 64 bit DXi, something is not installed correctly. I have just installed 8.5 64 bit on my new laptop and it worked straight out of the box.
VELOCITY - See my comments on nPulse

I see that a few of the synths came with Project5, so I guess I can safely ignore those.

Was Pentagon I included in Sonar 8.5?

Tripecac


Was Pentagon I included in Sonar 8.5?

Yes.
 
 I am wondering if you have an older version installed. I suggest you uninstall it and reinstall from the Sonar 8.5 disk.

I re-installed Pentagon I from the Sonar 8.5 DVD but it still does not work.

When I double-click on it in my plugin list (on the right side of the plugin manager) I see that it doesn't have a filename associated with it.

I don't see anywhere on the left (in either the DXi or VSTi list).  Toggling "excluded" doesn't help either.

Since it is a DXi, I don't need to include its path in the VST scan paths, correct?

I googled, and saw a recommendation to run Sonar as administrator.  When I do this, Pentagon I works.  There doesn't seem to be a workaround to get Pentagon I working in normal user mode.

I always run sonar as admin...