Why not build_as_arm, actually?

Hi all,
As far as I understand, the Symbian 9.1 SDKs build non-kernel code for 
thumb, by default (when building either armv5 or gcce).
There are explanations how to override this default and build for arm, which 
I suppose would be faster and possibly even smaller (if my application uses 
32-bit values everywhere then manipulating them through a 16-bit instruction 
set is probably more complicated).
What I haven't found is an explanation of why thumb is still the default, 
and why I wouldn't just build my application explciitly for arm.

Somewhat similarly, for pre-9.1 platforms, the default build specified 
everywhere is armi (interworking with thumb) rather than arm4. I assume arm4 
would again be a bit more efficient than armi, since it doesn't need the 
"change instruction set" instructions. But are there really devices
out 
there on which I'll need to interwork with thumb code, making armi the 
better choice?
(The application I'm working on uses only Symbian OS DLLs, I don't need to 
interwork with any 3rd party software)
And if the default for pre-9.1 is arm, why is it thumb now?....

Explanations would be welcome :)
TIA
Uriah