Nice analysis, Hypersmurf.
Yet another way of considering this is to evaluate the likelihood that such items show up when using the random treasure tables in DMG chapter 7 -- tables which not many people (I think) realize are statistically pretty stingy with treasure and magic items.
Consider (with some approximations):
- +5 enhanced armor and weapons can only ever show up as "major items" (Tables 8-3 and 8-10).
- For all "major items", 10% are armor and another 10% weapons (Table 8-2).
- For major armor, about 33% (19/57) are +5 enhanced (Table 8-3).
- For major weapons, about 22% (11/49) are +5 enhanced (Table 8-10).
- Hence for all "major items", the chance of a +5 enhanced armor or weapon is about: .33 x .10 + .22 x .10 = 5.5%.
Then, if we look at the treasure table (Table 7-4), we see that "major items" are in fact quite rare, starting at a 1% chance for EL 10 and working up from there. For the following, I assume all encounters are at the party's level, and multiply by the 13.33 encounters expected over the course of that level, to see the chance of encountering any +5 item at any time during one party level:
- EL 10: listed 1% for 1 major item; so .01 x .055 x 13.33 = 0.73% chance for a single +5 item any time through 10th level.
- EL 15: listed 10% for 1 major item; so .10 x .055 x 13.33 = 7.3% chance for a single +5 item through 15th level.
- EL 18: listed 20% for 1 major item; so .20 x .055 x 13.33 = 14.7 % chance for a single +5 item through 18th level.
- EL 20: listed 35% for 1d3 major items; so on average .35 x 2 x .055 x 13.33 = 51% chance for a +5 enhanced armor or shield throughout 20th level.
There are some minor approximations made in there, but nothing I believe changes the final conclusions (e.g., it considers only armor/shield enhancements bonuses -- not adding in armor/shield special abilities, not counting +6 rings or bracers , etc.). Using solely the DMG tables, there's only about a 50/50 chance for any party to find a +5 item throughout the entire 20th level of a party's adventuring career.