Alternatively there could be provided a shortcut or intent to enable/disable TTS. Or it could be added to the tasker plugin.
Then all users can define their own trigger through a automation app like Tasker, MacroDroid etc.
Btw: Feedback to TTS feature in AquaMail, personally, I will never use it.
My initial reaction to the TTS feature was: "That's cool and useful!". My second reaction was: "Umm.. when would I need it?..." (And how can I use it efficiently?)
After hearing the idea about BT, - I thought that if that were enabled (as an option), - I'd be using it.
Kostya: In the US, and especially away from a handful of big cities, car is the main if not the only way of daily transportation. In some (many?) states, it is illegal to drive and talk on the phone, unless it is a hands-free way (via a speaker phone or a BT). And many modern cars, even in more-or-less basic inexpensive models have BT built in. And a ear-phone has been very common for well over a decade. I know the situation is rather different in Europe...
Kostya, and all those users who are using BT for something else could turn that autoswitching off.
The same way as people switch off built-in browsing, and what's not.
nica: I had thought about Tasker after reading Kostya's definitive "no" response, but...
... in my present phone (Motorola Droid Razr M), there is Motorola's own built-in "Smart Actions", which I programmed to switch off BT and GPS when I am at certain locations (at home), to switch off the phone sound for the night time, etc... So, I doubt I'd buy Tasker just for this feature. Besides, I don't want yet another app that would be clogging up already overloaded phone (RAM, internal storage, already rather long startup, etc.).