Alot of people solo level and group for raids and R10s. But even with R10s alot of content gets skipped because people are mostly farming rxp with the easier and faster quests.
When solo-leveling I'll usually use a dual box buffing bard - have them at all level ranges and keep them parked without leveling them up. If I am grouping while leveling I don't use a dual box unless with someone I know that is ok with it.
For group raids I don't even consider bringing dual boxes except a few fringe cases where we are short-manning and others are also bringing their dual boxes. I do solo raid with dual boxes but mostly easy stuff - vod, hound, tempest spine, fire on thunder peak, etc and some epic raids for tokens, mats and collectibles (never seem to have enough of the epic level mats). It's a way to get threads, runes and sparks on my alt accounts mostly.
For level cap end game reapers I like to group with guildies to 3-man R10s, but there has been alot less interest in it lately as people are busy with real life so I've been soloing more or running with guildies that prefer mid skulls to R10. When I solo I bring 5 alts to add reaper points to some of my raiding alts on other accounts - and to have a better chance at items with reaper bonuses. 75 fragment is alot, but changing my odds to x6 does help quite a bit.
R4 is kind of my netflix binge watching default, but I often will go R6 and with the multi-year beefing up of melees I have been working on making R8 my new solo level but it's a work in progress - some quests are easier than others for that. For me R8 now feels like R4 did when reaper was a new thing.
As for people that have issues with courteous dual boxing (not bring them to raids, public groups, etc.) - its' their issue entirely as the argument against solo dual boxing has no real merit other than typical salty complainers that dislike everything that doesn't match their way of thinking.
The positives are obvious - it has to generate more profits for SSG and increase player spend. It allows people to adjust when friends and guildies aren't available and even for shortmanning - I enjoy shortmanning with others that are bringing dual boxes. People that want to run with me while dual boxing typically make out like bandits as I pass them all the loot without reaper bonuses.
Dual boxing is just a way for people to solve problems that would otherwise make the game less fun. In general I think people that are problem solvers rather than complainers enjoy this game more and progress better.