Adding yourself as a joint (co-responsible as you said) responsible party will not help your FICO more than AU. So, if your only concern is FICO, you all ready have that benefit.
The only possible benefit of adding yourself as "joint" is to allow the account to be considered as a credit reference, in addition to FICO factor.
AU's are not financially responsible, and therefore they will not be considered as a factor toward your personal credit experience nor as a credit reference when being considered for new credit. They will count in FICO calculations of utilization, number of balances, etc.
Most people on this board advice not to become a joint account holder under any circumstance. However, depending on your personal circumstances, goals and objectives, you may decide that one joint account is more beneficial than the potential risks.
I think it best to say, do not have all of your credit tied up in joint accounts and do not only have AU credit. However, I think it is too rigid to say never have joint credit. If you have 6-8 accounts and 1 or 2 is joint, you still have adequate personal credit resources to offset the risks of death and divorce of spouse. But you do need to manage the downsides of your credit should your spouse not manage this account. If that account is mismanaged, you are holding the proverbial crap credit bag.
I think it best to say, only use joint accounts where the benefits far outweigh the downsides and do not have all or majority of credit in joint accounts.
I'm not arguing with Hauling and the others in this thread. I do generally agree that joint accounts should only be used for a strategic purpose and only under certain circumstances. They have long term, irreversible consequences that must be considered, mitigated and managed.
