Morghase used her ability to wave a handkercief for a long stretch of time to attract attention to the building in which she was trapped with Faile and co. That's a great deal more than Egwene could do with dosed with Forkroot (she says she could swat a fly and it wouldn't feel it).

Sorelia may have been just as strong as Morghase, but better trained. The Tower didn't bother because it thought Morghase's ability was useless. Sorelia learned, allowing her to be part of circles reliably, and thus a part of the Last Battle. Its a true testament to the utter uselessness of Tower training for those they considered too weak.