Bookworm,
having read the thread about asking for turns, I don't get the impression you seem to have that ATCOs don't understand the implications of weather.
Every ATCO I know will give a turn unless there is a very good reason not to. However pilots should ask INCAS, but at the very least they should inform the ATCO that they are turning (asking is always better - after all the weather radar gives a long enough range to give adequate warning).
If a pilot was to turn without asking or saying anything it could have very serious implications on the safety of several other aircraft. ATCOs do not monitor individual aircraft 100% of the time, they make plans, implement them and expect adherence. Therefore pilots cannot expect ATCOs to notice the minute they deviate from their clearance. A clearance or instruction is not done in isolation, it is done taking other aircraft in the vicinity into consideration.
In a previous career (flying) I was involved in a MAYDAY due to severe downdrafts during a low level mountain flying exercise - I know what weather can do from a pilots perspective, been there, done that, got the T-Shirt and at one point during that flight, didn't think I'd have the chance to wear it!
That said, I would still expect pilots INCAS to ask before turning. ATCOs would have to have a very good reason to say no and would have to justify it if the pilot MOR'd the incident.
It's a team game, but neither side can operate effectively if not given the information they require!