I work in a small firm. I did part 1 and 2 of the AITI but never finished. I thought it would be of benefit. It became clear that it would not be of great benefit to me to have the formal qualification. - at least not in the sense of being able to show the qualification and get paid work for it (and when this became apparent, I didn't bother going on to do part 3 - tho I sometimes regret this decision). In my youth, I resented that colleagues would wheel me in to the room to review the tax aspects of a property deal or give tax advice to a client 'on the hoof', but would not see their way to invoicing for this service as an added value item.
In pure money terms, it probably doesn't pay a small firm non-specialist practitioner for the time it takes; In concrete terms, I doubt if the fees I have billed (specifically for tax advice on its own ) would amount to 7,000 in any given year (and zero in many years).
However, over the years, my interest and expertise in tax matters has certainly stood to me. You get a better class of work if you are known to be expert in the tax consequences of transactions. I have housebuilder clients who will never leave me for a cheaper service - the tax awareness is certainly something they appreciate. They might, of course, leave me if service standards slip, but that is a separate issue.