Not so fast, there is NO responsibility for ALPA to represent or provide legal resources to a non-member in a disciplinary hearing. The same is true with ALPA aeromedical resources, lose your medical, go find your own doctors to deal with the FAA. There are other ALPA membership benefits that non-members do not receive

You are correct that ALPA contracts are "closed shop" meaning that the contractual language applies to all in that category and class. Because ALPA negotiates the pilot contract for the category and class, non-members are covered by and benefit from those efforts (pay rates, work rules, retirements, etc.). Non-members are billed a "contract maintenance" fee which is at a different rate than membership dues.

Professionally, there's no difference in treatment when operating the aircraft safely but conversation is limited only to operational items. Interpersonally, it appears to be a very lonely work existence for non-members.