Couple of thoughts:
 
I know some synth functions (notably MIDI Out) will not work if there's no audio track receiving the synth's audio output (either as input, or via th FX bin). That could be why you couldn't get Geist's sampler to work until you put it in an FX bin. 
 
You can't insert a synth in the rack and then route its output to an FX bin. You insert it in the FX bin as you would any other VST, and then it automatically has a presence in the rack.

I was going to make feature request about it. Now you can't send audio to the synth in the rack, if you have not put it in fx bin previously. You can, but only if it has more than one audio inputs, in this case you will see them (starting from 2) in the sends and outputs menus (as sidechain inputs). It seems weird that you can't do it with main audio inputs of VSTis.

I don't think you can route audio into Geist without using the Splitter plugin. It really is quite simple, you just insert the Splitter on the track that you want to sample and then select the Splitter as the input in the Geist sampler. 

See OP. Have used splitter before that works. If you add geist to the fx bin of an audio track that is playing sound it does work as well. However at the time it didn't trying to work out why it didn't. If the scenario comes up again will look deeper. Cheers..

I have not used Geist so not fully clear what the problem is here. Most synths do not have an audio input. Does Geist have one?
Any synths inserted into the rack are considered "sources" so there is no audio input to them.
Inserted into a bin they do have track data streamed to them (assuming the synth has an input)

It does (as well as ability to resample its internal busses), but the only external input built-in is labeled "external." It has an included FX plugin called "Spitter" which can be put on any audio track (can have multiple instances), but the sampler only records one source as a time.
 
I am still unsure of this setup, since Geist is intended as an instrument (will accept multiple MIDI channels in, and outputs 32 mono channels of audio). Internally it is not intended to be an "effect" so not sure how it would route itself in this situation.

Noel Borthwick [Cakewalk]
I have not used Geist so not fully clear what the problem is here. Most synths do not have an audio input. Does Geist have one?
Any synths inserted into the rack are considered "sources" so there is no audio input to them.
Inserted into a bin they do have track data streamed to them (assuming the synth has an input)

Geist has an input (it has a record sampler) and it works most of the time without splitter plugin . I need to supply better steps for me to nail this issue.
The only clear problem is post #8 right now. Cheers.

Noel Borthwick [Cakewalk]
I have not used Geist so not fully clear what the problem is here. Most synths do not have an audio input. Does Geist have one?
Any synths inserted into the rack are considered "sources" so there is no audio input to them.
Inserted into a bin they do have track data streamed to them (assuming the synth has an input)

This model make sense only for workflow when it is not actually a synth, but midi-controlled effect. Otherwise, if it is synth that support audio input as additional audio source, it seems more natural to use "sidechain" model, and allow tracks to set output and sends to this input. Without it, it is not possible to set synth properly and only after that decide if you want to add audio input to it. Now in this case you have to go away from familiar workflow with synths and synth rack to do it.
Another way is to make it possible to drag and drop synths from synth rack to fx bins, because current functionality is pretty limiting.

Noel Borthwick [Cakewalk]
Any synths inserted into the rack are considered "sources" so there is no audio input to them.

Actually there are audio inputs, if it has more than one audio input. In this case it appears in outputs and sends destinations.

Yes any secondary inputs will show up as a sidechain input that you can route tracks to.
So what is the problem here in this case - you want to send to the main input?
 
The main input is not useful for synths in the rack since there is no way to send track data to it.
If you need to send track data to the main input, then the  synth should be patched as a "transformer" inside an fx bin. You can still use it as a synth by sending MIDI to it.
 

Noel Borthwick [Cakewalk]
Yes any secondary inputs will show up as a sidechain input that you can route tracks to.
So what is the problem here in this case - you want to send to the main input?

Yes, it is my feature request, seems very logical to me. Of course it would add more staff to outputs menu, but you've said that some changes are going to happen there.