Two problems with __wt_evict_file_exclusive_on:

First, WT_SESSION.rebalance (__wt_bt_rebalance turns exclusive access on, but doesn't turn it off, so after a rebalance, the tree won't participate in normal eviction.

Second, michael.cahill notes in BF-1841:

I have some doubts about the way __wt_evict_file_exclusive_on / off are structured: we want to be sure in some operations that eviction is excluded for the whole operation. It isn't enough to know that eviction was disabled when the operation started, which is all we can really be confident about at the moment.