Some cossie's had roller bearings in the arms, but these can seize up/wear out, then it is much easier to put poly in, but some purists like originality on a extensively modified cosworth, no I've never been able to figure that one out yet either!!
Absolutely nothing wrong with poly on all the points, a poly bush will give a vastly better location than a shagged sticking 20+ year old roller bearing.
The std sapphy suspension design is shit, that's why they self steer at the rear, made a lot worse with soft location from rubber bushes and the badly thought out diff hanger.
The sierra doesn't use trailing arms further adding to the problem, as the arms are semi trailing any upward movement of the wheel causes not only a camber movement but also a toe movement.
Poly bush the lot, then add a poly diff hanger, or you can get the std diff mount poly filled for £20-£25, these seem to work just as well, and look more ford like for the bufties.
It really will transform the car, then add a 22mm roll bar it really is a totally different car, you will get a fraction more vibration but nothing that you don't forget about as it's so minor, but well worth it for the tansformation, ford had to use whatever they had in the parts bin so that's why it wasn't as good as it should have been.
I didn't want to get ripped off with a escos rear arb(22mm) so had one made 22mm and adjustable for less money, and it clears my tow bar bracket.
tabetha