A sound hole pickup for temporary installation is great thing to have in your gig bag in case your main pickup dies at a gig or you want to use a pure acoustic guitar. If you don't get too hung up on my-guitar-only-louder there are a bunch of other benefits. I think the criteria are:
- Light, small, no external battery for quick installation and removal.
- Highly feedback resistant (no mics or Baggs floating coils).
- No hard materials touching your guitar to leave a mark (Baggs floating coils have no soft pads, small unpadded clasps on some DiMarzio models).
- Humbucking so that it works at gigs with nearby florescent lights or compressors.
- Active so that the inherent low pass filter formed by the coil of a magnetic pickup (worse for dual coil humbucking) does not kill the highs (takes out the cheapest options).
- Classic great magnetic tone for single note leads.
- No piezo artifacts.
- Some loss of acoustic goodness finger picked (without a mic or a floating coil this is what you sacrifice).
- No 3.5 mm jack on pickup to fail (permanent cable, which takes out most of the more expensive options aimed at permanent installation).
The Fishman Rare Earth Humbucker checks all these boxes and might be the only one. To my ear and compared to my other guitar’s Mi-Si Trio (not an AGF approved K&K) I might like it a touch better. For permanent installation I would still choose the Trio for having nothing in the sound hole.
It really is a very nice sounding pickup besides being a very practical choice.