I'll go on practice dives with new divers. No problems at all. Particularly if a new diver is getting ready for an offshore charter I'll go diving with them to get them familiarized with procedures, their equipment, communications, and the rest of their skills foundation. Practice dives are the place to learn, and I don't mind spending time with new divers to help them along. This is where the whole "we were all beginner's once" idea comes full circle, IMHO.
I won't jump into deep ocean dives with new divers unless I've practiced with them previously. If I'm familiar with them, we've practiced together first, etc., then I'll dive with them on deeper dives. If not, there are too many unknowns and variables possible in the ocean, including how they might react should they have "issues", for me to be comfortable with them.
With respect to considering the expense of paying for charter boat slots or remote vacation dives in deciding to dive with new divers, IMHO this decision does not only come down to a cost/benefit analysis. There is a safety issue that should be considered as well. It's important to recall that any diver could die on any dive at any time - and it may not always be my buddy who needs rescuing. I wouldn't dive with an unknown new diver, on a vacation or a charter boat, who is obviously exhibiting signs of incompetence or inexperience - because if I were the one who blacked out underwater I'd want a measure of confidence that my new buddy could successfully respond to that emergency. If I don't feel that a dive buddy could safely perform the dive, for whatever reason, then IMO its best not to dive with them.
FWIW. YMMV.