When it comes to comparing Thai script and good transcription (i.e. transcription that fully and accurately represents the sounds of the Thai language), two things are axiomatic:
Taking these points in turn:
Transcription is much easier to learn than Thai script
To expand upon the first point, transcription can be mastered in less than an hour. There are a few new symbols that may need to be understood, and the pronunciation of others to be mastered, but once one’s done that one can pronounce any transcribed Thai word. For example, Thai tones are indicated completely regularly by the use of four diacritics (shown here with a) á à ǎ â plus no diacritic - a. In Thai script the reader has to consider, amongst other factors, the class of the initial consonant, the length of the vowel, the presence of any tone mark, and whether the syllable is live or dead, to calculate the tone.
As for consonants, Thai has 42 which the learner not only needs to recognise, but also needs to memorize their classes. With transcription there are only 20 consonant symbols which need to be known, most of which will already be familiar to English speakers.
Learning to read Thai takes a very long time. After perhaps 100 hours of studying the script the beginner will probably not be reading Thai with any fluency, but painfully and slowly deciphering it.
Transcription represents the sounds of Thai more accurately
than Thai does
Thai script fails to represent the pronunciation of Thai words at two levels:
In brief, Thai script fails miserably in representing the sounds of the language.
What does this mean for the learner? Most farang who start learning Thai want to be able to communicate in daily life – ordering food in a restaurant or at a street stall, buying a train ticket, greeting people and asking them about themselves. If they have to spend several weeks just studying how to read Thai before they can start communicating with Thai people, they will become discouraged. I think it’s noteworthy than when I studied Thai at a school in Bangkok, virtually all the farang students dropped out after three to four months. They had learned a lot of phrases using transcription which they could use in their daily lives, but either had no interest in learning to read Thai, or found to do so too challenging. (In contrast, East Asian students were typically much more persistent.)
In practice, if students aren’t provided with a transcription scheme, they will typically invent their own which is most unlikely to be as accurate and consistent as schemes such as Haas and AUA.
One bonus of using transcription is that an unfamiliar word one hears can easily be looked up in a dictionary that has a transcription index. If one uses a Thai dictionary, then there are multiple possible spellings that need to be searched because of the high level of redundancy in Thai script. (For example, a simple word that sounds like tham could be written ฑำ, ฒำ, ทำ, ธำ, ฑัม, ฒัม, ทัม, or ธัม. (And that assumes you heard it correctly in the first place.)
To summarise: transcription is a tool which can help the learner make rapid initial progress learning Thai and can provide an accurate representation of how Thai is pronounced.