As tealamutt said, embryo transfer is extremely expensive. Why did your trainer suggest it? It is done for vairious reasons however. An owner might wish to breed, but not take the mare out of the show ring. An extremely good mare who has produced excellent offspring in the past, might be a senior now and the owner might not wish to put her through a pregnancy. A mare might not have carried offspring successfully in the past, to full term.
Re. the Gypsy Horse (Vanner) owner, who has pumped out loads of identical embryo transfers each year - well in my opinion, that is pure greed. And the majority of Gypsy Horse breeders in the US, are not at all in favour of the person in Canada, using Mules as moms, when he has plenty of Gypsy mares who could carry the baby of another.