Not enough seed is your playstyle... if you dont have that much, you dont. ^^ Plants grow slower in the mod.
I think the job cancellation spam is not related to a lack of seeds. I have plenty of seeds and still get that spam. Here's what I think to be the reason:
The "store seed" jobs makes a dwarf pick the seed barrel, then venture far down to the dining hall, pick up the seed, bring it back.
If, while dwarf is hauling seed barrel to grab seed, a planter wants to grow with hauled seeds, the job will get cancelled. As the seed is literally not there - one dwarf ran away with it!
I really don't like the new hauling system at all for various reasons, the job cancellation spam is one of those reasons. I wish toady allowed us to revert to "traditional item stockpiling jobs". It's also way less efficient when it comes to farming. Once a plant is grown, the farmer goes to the plant, marks it for "can be harvested", then he goes back to the stockpile, picks the barrel, runs back to the plant, brings barrel to stockpile. Like this:
Farmer --> plant --> stockpile grab barrel --> plant --> stockpile store barrel
While in the old version, the farmer would go to the plant, pick it up, bring it to the stockpile, like this:
Farmer --> plant --> stockpile store plant
Note: same tragedy with job cancellation spam applies to: brewing, processing plants, milling plants, pressing plants. Could also affect taming of herbivores.
Same goes for, say, smiths who need bars that are currently being hauled all the way to the smelter to grab a single new bar, thus the bars needed by the smith not being available at that time. And so on. Leather. Cloth. Gems. Wicker. All affected by this weird system of dwarves wanting to haul a whole bin full of stuff to store a single item in it, then haul it back.
Depending on the distance between stockpile and farm, this can cause you to lose a _lot_ of time, and almost prohibits the use of "only farmer harvests", as they'll just have too much to do to even think about planting new seeds.
Anyways, I dislike it. I don't think meph can change such things, though.