Answers from Sena Support:

- Why is H2DP mode not applicable with non-Sena devices although the H2DP profile is supported by the BT10?
The BT10 supports HFP and A2DP only as most standard BT devices support in the MOA industry. H2DP not in the mix is something the R&D Dept. choose not to include or what not and out of our control from the Warranty Dept. to know as this info is not released to us.

- What is the BT10 (as to my mind a real technical masterpiece) meant to be applicable for exactly?
Sena is a MOA (Motorcycle Outdoor Activities) company, so it's meant along the lines for this matter. 

- Is there a chance that a future firmware update might provide H2DP profile also usable for non-Sena devices as external Bluetooth speakers like UE BOOM 2 and others..?
Right now most of our products support Universal Intercom pairing for non Sena products, so that is it at the moment.
There is no scheduled firmware release yet but if there is an email will alert you (if registered) and to please check up on it too.

- This kind of useful Bluetooth voice to speaker application should implicitly be added to BT10 features..! What hinders you to do so?
Ok we understand your inquiry here but submit a feature requests to the developers here as we are not the development dept.:
We appreciate you bringing this to our Attention.

Ronald