My view is that this is a difficult debate to win. Usually, for Shi'i-Sunni issues, there is an attempt to "prove" that certain practices are or are not acceptable according to certain standards (such as certain texts). (The same is true if one is discussing between Shi'is and people who are not Muslims, or between Shi'is.) However, most people have their own preconceived ideas about what is acceptable.
Rather than taking this approach, in my view, it is better to promote a spirit of diversity and tolerance - an acceptance that different Muslims have different practices and ways that they live their faith, and this is one of them. That is, encouraging mutual respect for differences rather than trying to argue it theoretically. In general, I feel that these arguments come up due to a lack of tolerance in some streams of contemporary Muslim thought, and that lack of tolerance of diversity is our real problem, which manifests in different ways.
Other people think differently and consider it to be very important to argue these things textually and may provide a set of hadith to "prove" that matam is acceptable. You can find those arguments online easily if you search. In my view, they don't do the job wholly, because they are about spontaneous events that happened rather than an institutionalized, regular ritual practice, but nonetheless they can be useful in defusing tension if an appeal to tolerance and respect doesn't work.