experience and portfolio are both important, but
experience should come before portfolio
wedding event is still an event, but is much bigger scale then many types of private events, eg birth parties.
ones can gain experience in many type of other events, but weddings is once in a life time event (at least to most of the people), so, must take great caution when shooting weddings.
there are many discussions about how to start, or get into wedding photography, I shell not touch on this here.
as for are the couple entitle to have a set of images cerated by the tap along photographers? Yes, they should have a copy.
This is a form of thank you and appreciation the couple for the opportunity given to the tap along photographer.
why should the couple say yes to let someone tap along, and if they don't gain anything later? beside, they may have to provide addition logistic to the tap along photographer too.
if the tap along photographer don't intend to give images to the wedding couple, is like someone do a TFCD session for a model, and don't give the model the CD of the images after the shoot.
if the tap along photographer don't think he can produce usable images for the wedding couple, than he may try to polish more on his photography skills first before try it on other people wedding.
.