From many older match commentators such as Buddy Hall, Grady, etc... they wanted to have 13 as the pro tour races, they thought even 11 was too short. I have a feeling that is about what I would want also to keep random variances such as a high break and run count or a few lucky rolls (dry breaks, lucky safes, etc...) dictate the match. If someone runs 5 racks or gets a few dry breaks in a race to 7 or 9, that is tough to get through, if it's a race to 11 or 13 the other player has more of a chance at the table to get through those with skill, or some luck of their own.
For the average "good" player in a league, say high C or B level, a race to 7 is good, playing even. I feel playing a race to 5 I can overcome a few unusual things happening that caused the loss of game. For example I was playing someone some 9 and 10 balls games just for fun, he won the first 4 games but that was due to me leaving him the last 2 balls to shoot in. I won the majority of the rest of them, probably 90% of games rest of the night. If we were in a race to 3 I would have lost already. Race to 9 even giving up the first 4 I would have won.
There are a bunch of interesting comments in this thread, including yours.
For matching up just two players, would playing x-sets instead of one race-to-x be more equitable? What would be a reasonable structure? How many sets, and how would each set be structured? Thinking more of two good amateur competitors but comments about how pros would do it or have done it would be interesting, as well. Maybe money games have done this a lot?