Don't think of it as "losing a season of crops" but merely transferring it to a different field.
For ultra simplicity's sake
Say a farmer can plant 10 plots in a season
Therefore, a single farmer could keep a single 10-plot field fully planted without going idle.
That also means the farmer could keep two 5-plot fields fully planted.
Now...the season turns and the farmer stops planting field #1 and #2, and starts planting #3 and #4. 3&4 are also both 5-plot fields. So the farmer, once again, spends the season planting 10 plots, 5 in one field 5 in another.
Now lets compare this to a simple 10-plot field growing plump helmets all year.
Spring:
Plump Helmet farmer plants 10 units of plump helmets into his one field.
Total Plots Planted= 10
Rotation farmer plants 5 units of sweet pods and 5 units of quarry bushes into two fields
Total Plots Planted= 5+ 5= 10
Summer:
Plump Helmet farmer plants 10 more units of plump helmets into his one field
Rotation farmer's first two fields go fallow. He plants 5 units of pig tails and 5 units of cave wheat into his third and fourth fields.
Total plots planted by plump helmet farmer over two seasons = 20
Total plots planted by rotation farmer over two seasons = 20
The difference is that the rotation farmer got 4 different varieties of plants while the plump helmet farmer only got one. The plump helmet farmer's field only takes up half the space as the rotation farmer's fields, but as long as space is not at a premium (which it normally isn't in soil layers) the extra space used by the rotation farmer isn't a significant downside.
Granted, you get fewer crops of each type and if all you want are plump helmets, of course the single-field plump helmet farmer is what you'll want to do.
As for what to do with all that food and booze after you've bought out every caravan? Well, part of me sees the "fortress wealth" value as my score. There is something to be said about carving out an entire mountain and storing more booze there than your local aquifer.
Probaly more to do with labor time waste.
The time urist is spending wasting seeds, is time he could be using to make socks.
This is EXACTLY about labor time waste. I know that forts can easily max out their seed counts and having a few go to waste isn't a disaster. But the time it took that farmer to haul the seed and plant it is now wasted if they rot on the field.