On the Uileann Pipe thread this question was raised. Let me weigh in on it in a new thread so there is no thread creep...

ALL people who speak a language not shared by others in a deel tend to use that language among themselves during a transaction, anyone who does not believe this - come to New York, where there are at least 40 some languages spoken and ALWAYS in a store when price is discussed folks behind the counter use their native tongue, this does not mean that Bangala, Cantonese, Gaelic, Russian, Ukrainian, Finnish, Norweegian, Vietnamese, Korean, Macidonian, Romaness, Rumaninan, Italian, Hungarian, Japanese, Croatian... etc... are "secret" languages or "cant". In fact it is likely that the root of Shelta is a pre Celtic Irish language.
Now, it has been my experience, both in the US and in Ireland, that Travellers when one is a friend to them, not a seek out the odd culture ethnic tourest or dectective, are very free to teach their language.
In the United States, I have found a highbred language which mixes Shelta with Romanichal (Scotish and English Romani language closely conected to the Romaness spoken by Vlax Roma) for example the word for dog Zhucle in Romaness is Yukle, as opposed to the Shelta word which is not related.
So, though I have great fondness for the Gemelches, I would disagree with that observation, which I find remarkable when everyone who has taken time to get to know Travellers adds some words of Shelta in their work... some secret.
All the best
Larry