Blacksand:
You need something behind the spiked wheels. My planter was setup with a trailing 'firming' wheel, that does the job, it is basically a rubber tire that follows behind the spiked wheels, under light spring pressure (that was how I got the planter, if I were buying the parts myself, I'd probably go with the chains).
Anyway, you need something behind the spiked wheels, because in a dry year, if you get a few days of low humidity, with a wind, the holes where the spikes went, will cause the soil to dry out quicker. Anything that kind of fills those in, wheter it be a tire, or a chain drag, or whatever, will help preserve moisture.
I can't say about sidewall compaction in wet soil, because that is almost never our problem. Our problems stem from trying to close a trench in no-tilled wheat ground (which can be quite hard in the top couple inches) and preserving the moisture we have (vs, fighting mud). I can say, that from my very limited experience in wet soils (we are talikng the little wet spots in the field) that the spiked wheels tend to 'pinch' the soil together tightly where the spike went, and not so much in between the spikes, whereas the standard wheels tended to form a little ribbon of packed soil, on either side of the trench. The firming wheel, or drag chain, tends to help even out the ground surface, between those little pinched areas, and level off the little bumps and holes the teeth make.
I can say the ground behind the spiked wheels is less prone to crusting. In fact, we sold our old rotary hoe a few years ago, because it just sat there, unused since we got this new planter.
