Prophilaxis and Rehabilitation of Post Stroke language problems
If toki pona is somewhat like computer programming code and algebra, and if a stroke spares the ones ability to write code, and if the effects of the stroke are largely limited to abilities of speech, then toki pona might be useful as a way to express thoughts without relying on the parts of our brains that normally process language and instead rely more on the part of our brains that process math.
For much the same reasons, toki pona may be useful in dementia or brain cancer. In dementia and brain cancer, skills are lost at an uneven rate. Mutlilinguals lose all their skills last. Knowledge of any second language is an insurance policy against this eventuality, toki pona is an inexpensive "policy" in terms of hours it takes to learn.
At the moment, it would be more prudent to learn toki pona long before a stroke. After a stroke, one will probably still want to make an effort to rehabilitate their ability to speak English and it isn't immediately obvious that the effort to regain English is greater than the effort to learn an artificial language.
25%-40% of stroke survivors have aphasia, 100,000 people get aphasia a year in the uS and 1 million have aphasia now.
http://www.med.umich.edu/opm/newspage/2003/aphasia.htm
Communication for disabled children who can not speak or sign.
English is ill suited for pointing based language, i.e. language where you can only communicate via pointing. The alphabet is small, but pointing to all the letters for a sentence is excruciatingly slow.
Bliss symbols in one of the more famous systems created for this purpose, but there are many other competing systems. Toki pona has the advantage of only requiring a ~125 word symbols, 35 syllables and 14 letters or 174 symbols total. If the toki pona community continues to innovate scripts and short hand techniques, toki pona could potentially become more efficient than pointing to the English alphabet. As an added plus, this proposition is eminently testable by determining if equivalent phrases in English and toki pona require more keystrokes.
Another advantage toki pona has over some systems is that the language is rigidly isolating. Symbols do no inflect based on changes internal to symbols-- a technique common in Blisssymbols that some children and adults found difficult.
Adult onset dealfness.
English is ill suited for signing-- it has a heck of a lot of words that need to be signed distinctively. ASL is perfectly fine if you know it, but is a natural language, so it poses the same challenges of French or Spanish in that it takes 500+ hours to learn. Signing each letter is slow and tedious as is writing everything down. A lanugage of a small number of isolated morphemes would be easier to learn for a deaf adult and the hearing community around them. Or they can learn ASL or signed English.
Depression.
Depression is usually treated with a talk therapy or anitdepressants. The antidepressants either work by modifying brain chemistry or they work via the placebo effect.
It's also been speculated that antidepressants work via a neurotropic effect. In that case toki pona could be effective as a way to encourage neuron growth.
The other possibility is that a language that makes it difficult to think unhappy thoughts and easier to think happy thoughts would help the symptoms of depression. This pivots on the various forms of the Saphir-Worf hypothesis, which is considered difficult or impossible to test and certainly the linguistic community is no where near the level of consensus comparable to the level of consensus one sees in the medical community when asked about the relative efficacy of leeches, tin hats or pennicillin.
If anti-depressants work primarily through a placebo effect, then toki pona is an excellent palcebo because conclusive evidence of it's efficacy or lack there of doesn't exist--yet. So anyone using toki pona will get most of the benefit of the placebo effect, if indeed that is the main mechanism. Furthermore, linguists won't stop aruguing about Saphir-Worf effects for years.
http://en.wikipedia.org/wiki/Brain-deri ... hic_factor
Preventive Care
There are a few charactistics of languages that make them more suitable for prophylaxis than for care after a health event.
Languages need to exist in a community for them to be useful. If a language user doesn't have at least one domain in which to use a language, it won't be used. An unused langauge can't possibly have a therapeutic effect any better than sitting through a Spanish class or underwater basket weaving class, for that matter. That domain of use and the community of friends who use the language need to be established in advance of a health event for a constructed language to be pragmatic.
It takes 500 hours to learn a language, possibly fewer for toki pona. In the case of stroke, brain cancer--there might not be the time or mental resources for taking the time to learn a foreign language. Moreover, the surrounding community would also need to get up to speed on the language.
Why an artificial language and why this one
For some of the purported effects listed, one could just as easily study Spanish or Chinese. Toki pona has the advantage of requiring fewer hours to master. Esperanto (and maybe even Piraha), Basic English also make claims of being unusually "easy" languages. The disadvantage of Piraha is that materials for learning Piraha are extremely rare and aimed at professional linguists.
The state of the art in the design of easy to learn languages has been making progress over the last few hundred years, so there is likely a trade off between ease of use and the size of established community.
As noted before, languages don't exist without a community. Every community is different and the community one finds themselves in will in large part determine what language can be spoken, regardless to their merits.