I used to consider voyage plans just something to meet the SMS requirements but I've come to see them as very useful.
Mariners have long planned voyages without formal voyage plans. A voyage plan just ties together the various elements we've always used and provides documentation.
For example a chart drawer with the voyage charts is listed on the plan as a list of chart numbers. An electronic list of waypoints can be referred to on the plan by the name of the file or by printing out the waypoint list and attaching it.
The SMS might have a template to use which can be inconvenient but work-a-rounds are easy. For example mine has a section for tides in both the arrival and departure ports but we print out tides and post them. In the section for tides the template now says "See attached sheet" After the voyage we take the tide sheet down and staple it together.
Waypoint for Windows has some features that can be used as part of a voyage plan. The "notes" can be used to save the boiler plate description of the voyage. It also has a section to list the chart numbers.
Old voyage plans can greatly speed up voyage planning as changing , dates, voyage number and a little tune-up are all that is needed to update.
I correct voyage plans during the voyage (vhf channels used by port control etc) by hand then the 2nd mate uses the corrections to update for the next voyage . I save the files using the voyage number so the most up to date one quickly found and retrieved for use.
Bottom line - voyage plans don't add to workload, they reduce workload.
I too initially gave the idea of a formal voyage plan little value, however and like you I have come to find them immensely valuable and useful (with a few workarounds). Presently the ECS we're using ( like many others) provides a means to print out all way points, courses and distances, dock to dock. I applied myself to be a bit more fluent in MS Excel and found that a good plan could be cobbled together and actually give me more than the S.M.S. required paperwork. My ETA's have improved dramatically and my UKC calculations are more realistic. I can factor in rise or fall of tidal height, average out speed over certain runs (and adjust U.K.C.) as well as nail down an E.T.A. to within 15 minutes in a coastal trade situation. I have a catalog of regular runs that allow for a quick setup by plugging in the basic values of draft, speed and time of departure. I could add ton more data to the document but the mere fact that I have more than a basic DR plan to compare to my real time results helps quite a bit.
My moment of enlightenment came west bound across the Atlantic right after the Right Whale regs came into effect. Our arrival port got switched to Jacksonville and I was working on the ETA. I didn't figure I needed to look at the voyage plan because I've been to Jax I don't know how many times but I was scanning the old voyage plan for some other info and I realized we had forgotten about the 10 kt slowdown.
The voyage plan is a good tool for trapping errors.