Occasionally you'll see fundraising copy that reads something like this:
The rain slashed down like needles as the evening darkness spread, bring the fear of night to the huddled inhabitants of the homeless camp under the roaring traffic of the freeway.
That's (arguably) strong writing; at any rate, it's detailed and sensory. But would you write like that in a letter to your mother?
Or would you say it more like this:
The homeless people out there under the freeway are so hungry and cold. Won't you please help them?
The first example is "workshop" writing -- because creative writing workshops are where you learn to do it. It's a form of art, not a form of person-to-person communication.
It's stylized, specialized discourse. It doesn't come across as authentic communication.
Good fundraising is an authentic, human connection, not high-end art. If you want to genuinely reach people, tell stories the way people tell them verbally, not the way novelists write them.
Colloquial writing can be just as dramatic and powerful as workshop writing. But it sounds like a normal person is talking, not like a writer plying his craft.
And colloquial writing takes just as much skill -- more, actually.
Save your workshop writing for the workshop. If you're fundraising, concentrate on connecting with people on a personal, heartfelt level.