Now I am not the most experienced lizardmen player out there, but I have played other armies and I can ascertain that spears for saurus warriors are not all that great. Saurus to me are not meant to kill off a whole unit by themselves. They have a good amount of attacks at a good strength but have a low WS and low Int compared to most armies. The extra rank of attacks is not worth the extra points.
The width of a saurus unit in my opinion should be 5 or 6, this way they keep steadfast longer. In a horde formation you will be attacking more, but they will break faster. So if you have spears in a unit with 5 frontage you only get 5 more WS 3 attacks, and in horde you get 10 (which is much better) but as I said, horde is not the best formation for saurus. Not only are the extra attacks not worth it, they lower your survivability (albeit a small effect) with spears you do not get the 6+ ward save parry.
Saurus will probably not kill a unit by themselves since they will most likely be hitting the front of another large unit, and the extra attacks probably will not make as much of a difference as your possible steadfast and parry saves. They need to survive long enough until they get a magic boost or get support from another unit.