Hi!
In principle it sounds good but the surface plants would need to be buffed to compensate. At present the advantage of underground farming is that you can do it immediately since you get the seeds on embark. Surface farming needs a skilled herbalist and time before you get your seeds. That and at present there's more you can do with underground plants (compensated for there being more variety in overground plants and the seasonal nature).
O.o
You know, I am not quite sure whether we are talking about the same game there. As I mentioned several times across the forums, I am a die-hard surface farmer, and I have to admit that I approach that happy-go-lucky: My embarking dwarves have various skills, all at NOVICE level, including one or two novice herbalists. I simply designate an area for plant collecting right together with my lumber designation and at the same time build three or four 2x2 farm plots.
A little while later, I check on the farms if I can plant any plants that are directly edible, and if so have each farm specialize on one plant.
After that, I basically forget about the farming altogether, and I never had any problems with too little produce (my only problems were when I forgot to brew some of the plants and ran out of drinks, but the farming becomes efficient very quickly).
So, I can't really say I ever experienced any of the hardships/problems you are describing for surface plants.
Psieye: Well, I am personally fine with the way things are, but I wanted to voice my support for Sinistar before the shooting begins
Deathworks
Well, it's just my philosophy that embark dwarves should go all or nothing with skills. If I take Herbalism (and I often do), I max it out so I can get large wads of plant each gathering trip. Ok so it was an exaggeration on my part that it takes much longer to get surface farming up than underground farming, but provided you start with picks then to me it's still faster and easier to get underground farming done - whether you want advanced and reusable irrigaiton or not is when it gets longer. With surface farming, I need to make sure plants get brewed (which means making barrels, which takes time or embark points depending on if you're forging your own axe) and then go planting, easy sure but it takes time.
Anyway, indeed these are small concerns compared to getting more bugfixes out.
I'd like to say that the surface plants would need to be buffed a little, because underground farming has one big plus over surface farming: security.
Underground farms are surrounded by rock, have only the entrances you dig out, and are easily accessible by the rest of the fort. Digging through the soil is very quick for experienced miners, and provides good practice before they go out and dig through whatever mineral veins you're fort ends up in.
Surface farms need to be walled in to be secure from sieges, stealthy kobolds, raiding parties, wandering cougars/elephants/unicorns/skeletal carp, thieving raccoons and rhesus macaques, and are still vulnerable to flying menaces such as buzzards and giant eagles. Properly defending a surface farm takes much more effort and time than a subterranean one, even if built in an easily defensible location.
Personally, I enjoy fortifying enough space aboveground for surface farms and miasma free butcher and tanner's shops, but it is more of a challenge than taking a minute while you dig through the surface soil layers to get enough room for a plump helmet farm you can rely on the entire life of the fortress.