If Fayris has it, but at the same time she is unable to use it (various reasons), then it is imho a good thing to have one character being able to use it, as some kind of "backup". That's my personal opinion.
The use works basically like with all talents - if I recall this correctly - that you throw a 20-sided dice to meet the requirement, but if you don't, then you'll have the "talent points" to help you out to fill up the to number you need, so to say.
Plus, the higher the talent points, the better the result, at least in pen & paper TDE this is the case.