Yes, the Q17 is frequent, but not very reliable. Usually, when I wait for a Flushing-bound 17 at the 182nd Street and Hillside Avenue bus stop on an early weekday afternoon, I need to wait for approximately 12 minutes, and usually at least five other buses come before the Q17.
Once, my friends and I had to wait for 45 minutes. Granted, it was raining pretty hard, which might have caused delays, but four Q43's, and basically at least two buses from every other bus route came by before the Q17 did. It was quite unpleasant.
And yeah, the Q25 totally needs LTD service on weekends, or at least more service. The Q65 needs more service too.
Both routes get packed along Kissena Blvd. It's not uncommon for the bus to be so full, that people need to temporarily get off the bus in order for others to get on and insert their metrocard.
The Q17 pretty much empties along HHE and 188th Street, so I see why the it doesn't look so full in Jamaica.