I'll concede that it is hard to say if it is truly possible or not to use such complicated technology for recreational use, but I highly doubt it. The human mind, especially memories, are very delicate. The idea of putting an entire fictional life on top of yours and just walking away from it is laughable.
I too enjoyed "The Inner Light" but Picard just back on duty like it was nothing was dumb. I understand that TNG was during a television era when arcs weren't as prevalent and they preferred to stay episodic, but geez...
Anybody going through that trauma (or the one that O'Brien suffered) through would not just bounce back the next day or even in a few days.
Of course, we are entertaining a conceit. We can only determine the usefulness of such a technology according to how it is presented. Such neurochemical manipulation would probably have unforeseeable consequences in reality. However, we are basing what we write on what we see on the show. In that universe, it is possible for an individual to have a mental alternative lifetime and pick up where they were in their normal life, more or less. And let's not forget that O'Brien's suffering was by design, not simply an after-effect of the mind manipulation.
Technology like this sounds nice because people are assuming you can spend that time...learning a language or a skill but how? Would you live the life of a student for a few years? Wouldn't even that short amount of time not be disorienting? I mean, maybe it wouldn't be as bad as what Picard or O'Brien went through because they had no hope of going back to their old lives. They had resigned themselves to their artificial lives.
They had these lives imposed on them against
their wills. That need not necessarily be so. Perhaps an individual can choose
to sit down with a Klingon speaker for ten hours, day after day, week after week, until he or she gained proficiency in the language. There would be no resistance to the process. Moreover, the person could go in with the knowledge that the process would not be endless. That might mitigate some of the uncertainty that plagued O'Brien. Maybe the individual would need a social life as part of the program. That still would be compressed for time.