Validity table defines the range of the characteristic values for which the non-cumulative KF is valid.
Let us say your cube has 0calday, and it has data from 01/01/1999 to 12/31/2000. If you run a query and check (say its stock cube) stock balance per day per plant, it will calculate balances for each plant and show starting from 01/01/1999 onwards.
However, if you had a plant that became operational on 06/01/2000, you wouldn't want to see a 0 stockbalance for it starting from 01/01/1999 (or say when calculating the average, take as if stock existed from 01/01/1999) but from the date the plant actually existed ie 06/01/2000.
In this case, if your validity table had only 0calday, your report would show 0 stock for each day from 01/01/1999 to 06/01/2000 for this new plant (not a good idea), while if you defined it at plant level, you will see the report starting at 06/01/2000 while it will start from 01/01/1999 for other plants. This is what you want.
There are other variations of this, but this is a simple example where validity table has implication.
I am not sure I understood your question.
Are you saying 'country' is also a characteristic and you are getting additional data to the cube for these new countries? Or just that more data is now being added to the cube?
If the plants are more or less defined for similar date range, or are defined for the period for which typical reporting would happen -
Say plant 1 is valid from June 1999, Plant 2 from Apr 2001 .... and so on. This would be the situation where you need plant in the validity table.
However, if your users are typically reporting over last two years, and not many plants have been added in this period, and not many are likely to be added later, you can remove plant from validity table. This should generally not impact query performance. Only if users frequently report on the overlapping period (when the plant existed, and not existed) would there be some performance difference. Unless the no of plants is high (say 100 ) the difference in performance either way should be minimal.
This is my limited understanding ( ie to say do not go solely by it ).