Morrowind: With 10 major skills out of 30, there are 30,045,015 possible skill combinations.*
Oblivion: With 7 out of 21, there are 116,280 combinations*, 0.38% of Morrowind.
Hypothetical scenario A: With 6 out of 18, there are 18,564 combinations*, 15.9% of Oblivion, and 0.00006% of Morrowind.
Hypothetical scenario B: With 5 out of 15, there are 3,003 combinations*, 16.1% of Hypothetical scenario A, 2.58% of Oblivion, and 0.000009% of Morrowind.
Oblivion cut 7 skills from Morrowind. So, just cutting 3 more skills from Oblivion's already somewhat-empty palette will cut character possibilities by nearly 85% than what was present in Oblivion. Cutting 6 more skills from Oblivion will cut character possibilities by nearly 98% than what was present in Oblivion.
*Formula used to calculate combinations:
C = (n! / (r!(n - r)!) where C = the amount of combinations, n = the number of possible things to choose from, and r = the number of things chosen out of the possible things to choose from. The (!) means http://en.wikipedia.org/wiki/Factorial.