I have used roof collected rain water in the past but I would not
recommend it except in a pinch - lots of others on the list may
disagree. Even though many shingles are now fiberglass (and slate or
tile roofs down here), all of the dirt, dust, bird droppings, etc. make
for pretty dirty water. Also, in hot climates the sun on dark shingles
is going to soften the binding material which will leach to the surface
and wash off with the rain. I used to let it rain for awhile before I
started collecting water, which helps, but is still less than ideal. As
a matter of fact, I now have gutters on the section of my roof over my
outdoor plants to protect them from runoff.
FWIW, Tom in Fl