Are you using the interim plans to store progress information during project
execution? While they're fields that are up for grabs to use for whatever
you want, in terms of tracking progress they lack most of the crucial data
you really need for that. The interim plans only store task Start and
Finish dates and don't include cost, work, or duration information. The
baselines store work and cost data, for example, that the interim plans
don't. Baselines also show how the work is distributed over the duration of
the task while the interim plans only show aggregate values so saving an
interim plan at the end of each week doesn't really give you any true status
or progress information.
I think of interim plans as storing an audit trail of the last few
iterations of the plan prior to beginning work. The sorts of changes you
might record using them are tasks being added or deleted as the plan is
finalized, for example. But assuming I've read between the lines of your
post correctly, they are a very poor tool for monitoring progress of
execution over time.
I'd strongly suggest you review the methods of posting progress into tasks
as work moves along, along with Earned Value as indicators of progress on
schedule and budget. Posting actual work on the dates it occured and
updating the plan afterwards to reschedule work or revise estimated
remaining durations as necessary, using EV you can set any status date
during the project that you want and accurately say "Here's where we were on
the first of Nov, here's where we stood on the 7th, here's where we stood on
the 15th, etc etc"
