When an election gets more than 10 nominations, primaries are held to select the top candidates. Currently, primaries select the top 10 nominations so that no election will have you choosing among more than 10 candidates, but that can be awkward when you are asked to pick the top 10 candidates... out of 11. Spending three days throwing out one candidate sound both awkward and a waste of time.
I think it would make more sense if primaries were still triggered upon reaching 11 nominations, but then they should pick the top (say) 8 candidates. This would make primaries always meaningful (cutting down on the amount of choices to the most prominent/popular choices) but never awkward (you'd always prune at least a few people, and not just one or two).
As proof, I ran the elections of Stack Overflow, Programmers, Super User and Gaming, with and without the users that came up 9th and 10th at the primaries. The differences are negligible.
The full ordering of the candidates can be obtained by running the elections with 1, 2, ..., 10 seats. The 4th place candidate is the one that wasn't elected with 3 seats but is elected with 4.
To simulate what would happen when the 9th and 10th users in the primaries (marked yellow in the tables) would have been excluded in the elections, I used the "Withdraw candidates" option of OpenSTV, so that any preference for the two users is discarded correctly and the relevant ballots are updated correctly.
In no cases was the ordering of the remaining candidates changed by this, and only 50 votes from Stack Overflow (out of 4,269) were completely removed as the result of ignoring the bottom two candidates. I think it is fair to conclude that running an election with 8 or 10 candidates makes no practical difference.