I'm not a taxonomist but I actually started out ca. 60 years ago to become an entomologist. I eventually studied chemistry instead, to have some hope of being able one day to earn a living in science. I ended up a biochemist, watching the science of Molecular Biology being invented next door. So I'm in no way any sort of professional taxonomist! But I definitely know what a gene is.
I also get the notion that taxonomy is first and foremost trying to reflect the most probable genetic and evolutionary history of the living organisms which it describes.
The criteria for taxonomic levels like species, subspecies, variety, and form are different between zoology/entomology and botany. Animals rarely form fertile interspecific hybrids; plants very often do, since humans easily break down the barriers that in nature prevent species from crossing: calendar barriers, physical barriers, pollinator barriers, geographic barriers. Many perfectly good plant species hybridize and produce fertile offspring because there were other barriers to keep them separate as they evolved in nature.
In addition, the populations we see in nature may be -- indeed, most certainly are -- in the process of evolving. A species is not necessarily a static thing.
I think there are even differences in how botanists view "species" depending on the genus or family they are studying. Certainly, the ideas vary within a plant group between individual taxonomists. "You pays your money and you takes your choice!" I tend to favor splitting, in the hope (as a biochemist) that we can recognize genetic variations within a species with names.
At the species, subspecies, and variety levels, the key criterion is the existence of a recognizable reproducing population. Again, we have to go back to what we can observe in nature, "in the wild" as it is sometimes described. What arises under human aegis is not something that taxonomists are eager to deal with, understandably.
That mess will someday have to be dealt with, of course, as it is likely that in a few centuries nothing will exist on this planet that humans have not messed with. That thought saddens me, but it also appears inevitable in any realistic projection from our current situation.
Also, as far as I can see, there is no consistent functional difference between "subspecies" and "variety." If there is, I would gladly have it explained to me.
The "forma" taxonomic level is appropriate for recognizing individual genetic variants that occur within such reproducing populations. The occasional yellow or white flowered individuals in a population that is largely red or orange flowered, for instance, would be a yellow or a white "form." Those contribute to diversity within a species and seem to me well worth recognizing. We can realistically expect that someday someone will provide a detailed molecular genetic description of the various forms and how they each differ from the typical form. The same distinctions could apply to differences from one individual to another individual in number or length of spines within a population or a species of cactus, too.
For instance, referring the the yellow-flowered type of Clivia miniata as a subspecies is not appropriate. Yellow clivias should be referred to as Clivia miniata, var. citrina, that is as a variety, not a subspecies. In C. miniata, the yellow flowers are do to a simple mutation in one gene, a mutation that exhibits simple Mendelian inheritance.
Finally, as a non-taxonomist I think that taxonomy should be useful and should provide us with the descriptive tools we need to understand and if need be to preserve various species.
Note: This is essentially material I posted to the [ToocoldforCactus] list on Yahoo Groups. See that list for David Ferguson's excellent discussion of what should be named, and what should not, in botanical taxonomy.