At one of our users, we saw that they have been doing heavy GETs in their custom coprocessor to check if the key is present or not in the current. At most 99% of the time, the key is not expected to be present during the initial load as keys are expected to be random, but there is still some chance that there is 1% of keys would be duplicated. But in the absence of BloomFilter, HBase has to seek HFile to confirm if the key is not present, which results in regression in performance for about 2x slower.
Even in use cases like Index maintenance and "ON DUPLICATE KEY" queries will also be impacted without bloom filters.
As Phoenix is still used for GETs by the users (SELECT query with key as a filter). and we also have constructs that intrinsically do GETs like Index maintenance and
"On Duplicate key". So I believe it is always better to have a bloom filter should be "ON" by default as I don't also see any implication of it, even if it is not getting used.