Okay, well first off lets point out that the group can trust the Agamari.
So, good reasons:
1. Most of it is dead weight and useless.
2. They'll have to carry it.
3. No one is giving them extra pack animals for the purpose.
4. The chance of the group getting ambushed and lossing at least some of it is high.
5. If they don't leave it behind they won't be getting much help, which they will need. Guides at the very least. Not to mention sacrificial animals.
6. They can't land North of Tol Dura because the no tech mojo gets stronger nearer the polls. So they are looking at a hike to fortress and then weeks on foot further north, ergo they need good horses for the humans at least.
7. Admiral can confirm this, nay or yay, but if I were Loki I would use the Tarsai against the Agamari, ergo anything they loose will end up in Loki's hands and I doubt they want that.