Tells you about the origin of the language/ethnicity and its roots (we are talking about a linguistic ethnicity here).
Do you honestly consider Muslims to be Bengali in the cultural sense.? I dont.
I only consider our Indian Hindu Bengalis in West Bengal as Bengalis.
I would maybe classify the Bangladeshis that wear saris, have tilaks on forehead for women, wear bangles etc as Bengalis. But these are all Hindu traits, and the Bangladeshis that do so are being Bengalis,at the cost of being "good Muslims".
Hindus = Bangles, tilak on womens forehead, saris, language involved from a HIndu Sanskrit script. HIndu Bengalis in West Bengal meet this criteria. Muslim Bangladeshis don't. Thats my honest opinion.
Overall, the centre of Bengali culture is in West Bengal around Kolkata now. You guy sare free to go claim Arab culture
(or Afghan culture apparently as kobiraaz mentioned)