Not irresponsible as the aircraft was cleared into icing conditions. It was a military aircraft and had no restrictions on flying in icing as a result of no airframe anti-icing - not many do. I can't remember the exact wording in the manuals (close to 20 years ago) but it mentioned exposure to moderate icing should be minimised. It had engine and canopy de-icing. Both were subsequently upgraded, partly as a result of this and other incidents. It was a brand new aircraft and we were still learning about it.
My point - relevant to the thread subject - is that rain ice can form extremely quickly and is very dangerous. And airframe de-icing may not protect you.
Mansfield - thanks for the info. I shows that we are still learning.