I think HA filters would still be needed. But using long pass filters probably would not be necessary. Assuming a very dark sky.
I got some Oh-WOW-ga Centauri last night with the 15" at a Bortle 3 site (though it's straight in Phoenix' light dome). It was just a blob in the eyepiece due to horrible seeing that low in the sky. The device fixed that right up. The 642 cut through the seeing even better. That's the harshest IR pass I have, but I would have tried higher if I could. With the 642, I could see about 3 bands of seeing passing over the core like a flag waving. The three giant voids also showed up better. The shimmer of the stars was prettier without the filter, probably because of the seeing. I haven't tried the device when the seeing is actually good because the eyepiece views are literally stunning. I'll try to tear myself away sometime.
Some dark nebulae also respond differently at the edges. The 642 also lets in the h-alpha, so it allows some better contrast in reflection/emission regions like the Swan or Lagoon. The effect is usually subtle, but not always. Same for Bortle 2 skies, but even more subtle. For galaxies and globs, they tend to hurt, but may help darken the dusty regions sometimes. I have a 610 as well, but rarely use it. Special cases like in the zodiacal light or what not clearly help. That's not really light pollution, but it's not target either.