There is inherent value in cultivating moral virtue because people are moral actors who must practice moral acts lest they falter or be derelict in their moral duty when faced with a moral dilemma. People don't have time to read tl;dr philosophical works and people will be faced with novel moral dilemmas that don't have tl;dr philosophical works written about them yet. Thus, they must have the tools developed to fly solo.

To use an overly used hypothetical, whether one decides pulling the lever is the morally correct choice is a very different question from asking what steps must be taken to make sure that the lever actually gets pulled when the trolley is about to cross the track. Writing paragraphs upon paragraphs about how you should pull the lever means nothing if you abandon your moral duty to pull the lever when the time comes. Only virtue ethics seems to at least see this as a very practical problem (their solutions admittedly leaves much to be desired).

The question should be whether abstaining from certain commodities constitutes a form of moral cultivation that's useful as far as cultivating moral agents who can automatically perform morally good acts is concerned. I do believe it is useful in that regard. You say that boycotts only work within an organizational context, but how would the org even stick to the boycott if no one in it has practical experience with abstaining and every one within it does not see its material value? For the org to even vote on the boycott, there must be members of the org who have already privately abstained from the commodities in question.

To use chocolate as an example, the members of the org won't vote to boycott chocolate unless there's already a critical mass of members who have already privately abstained from chocolate or a similar commodity like meat and who can then use their critical mass as leverage to convince the rest of the org that boycotting chocolate is the right tactical thing to do. Without practical experience, the boycott would most likely fail because individual members lack experience to flexibly adapt their daily lifestyle to a new normal nor have the tools equipped to face mounting social pressure, things that they would have if they actually have experience privately abstaining.