Speaking as a reader, and not as a writer, my feelings are this: If I am reading your comic, and I start feeling like you are winging it, unless I am doing a review on it, I will stop reading.
Let me repeat that: If I feel like you are making it up as you go along, I will stop reading. The only exception is a story that I am reading just for the humour.
I am not alone in that, however most people don't have my background in writing and critique, so they will give reasons like 'too many plotholes' or 'the story wasn't going anywhere' or 'too many abandoned characters/plots' or a host of other reasons. It all boils down to the same thing, however, and that is that the story is not written completely, and the reader can tell.
As a writer, the best reason to do a script (and a complete set of thumbnails/storyboards is just another form of a script), is that you can see how the story looks from farther away than a single page. You can then do things like work on the plot, work on the character arcs, eliminate things that do not advance the story, make sure that the pacing is smooth and speeds up or slows down at the places you want it to, and most importantly, make sure that the story hangs together in plot, conflict and theme.
Now, the most important part. The first one is the rough draft. Not the final draft. The rough draft. The finest, most practiced writers in the world have rough drafts, so even if you are doing thumbnails or storyboards, it's a rough draft. Polish it up before you start drawing.
The readers will appreciate it.
Eagle
("Writing" can be done with just the storyboards/Thumbnails)