The word Baha'i has been spelled either with or without an apostrophe, depending on personal preference, ever since the first speakers of English and other languages written in the Roman alphabet became aware of this new religion about a century ago.
In general, Bahais with more liberal views of the faith have tended to be more likely to omit the apostrophe – and this tendency grew stronger since Shoghi Effendi, recognized by most Bahais as the leader of the faith in the early to mid 1900s, attempted to force all Bahais to use a standardized system of transliterating Arabic and Persian words into the Roman script. Thus, spelling Bahai without an apostrophe can be interpreted as a quiet, unobtrusive statement of personal freedom.
The apostrophe is actually a profound metaphor in this case, because it represents the Arabic letter hamza, which is pronounced as a glottal stop in the throat. The hamza in the word Baha'i is a stoppage of breath between the vowel sounds á and í. And what liberal, open-minded Bahai doesn't suspect that the breath of the Holy Spirit has somehow been stopped or blocked from flowing through Bahaism as fully as possible during and after the time of Shoghi Effendi and his "administrative order"?
Dropping the apostrophe from the word Baha'i can therefore be a powerful symbolic act of inviting the Holy Spirit to flow freely through the Bahai faith and declaring one's independence from the rigid, overly bureaucratic and authoritarian Haifan Baha'i administrative order. Knowing that the spelling with the apostrophe is considered standard and officially correct among Haifan Baha'is, writing Bahai instead of Baha'i is either consciously or subconsciously a stand for individual freedom of conscience and action rather than blind imitation of institutionally approved norms – without needing to say another word to "spell it out" further!
The Unitarian Bahai Association invites you to remove the blockage from your Bahai faith and let the Divine Breath flow unimpeded through the throat of your spiritual journey. You can say so much with just a minor change of your habits of spelling – and in this way signal to other Bahais who "have ears to hear and eyes to perceive," using a subtle technique of hikmat (wisdom), that you have left the "apostrophied" and restricted, restrictive religious attitude of many Haifan-loyalist Baha'is behind.