.stl is the default file used by most additive 3D printers because it was invented for that, and long enough ago (mid 80s) that there were almost no CAD modellers that used curved surface representations. I don't know if .iges had even got that far then. That being said, .stl is simple, universal, and because it's meant to be an output format, not a modelling format, you can increase the faceting resolution of your modeler. It works like a TC surface modeler because modifying it proportionally transforms the vectors between vertices, not the parameters of the surfaces between them, so it doesn't destroy their compliance with .stl rules because it doesn't change the relationships between faces, edges and vertices. Even in TC deluxe, use high-count polygons, and extrusions and cones of them instead of the native primitives to improve resolutions. It's not meant to be a modelling format because, if you boolean two objects with different mesh densities together, boundaries become hopelessly complex to try to comply with .stl file rules, which stipulate how many facets can share vertices and edges.
The ACIS kernel isn't that expensive, because TC Mac has it for US 130, so IMO, PC TCadders are being short-changed, even if MacFolk are donating arms and legs for nice hardware. There's even the OpenCascade open-source (free) kernel that could keep the price down and give .igs and .stp and b-rep curved surfaces.
Even if 3D printers could interpolate curves in x and y, they'd still be layered/stepped from the incremental lowering of the platen. There's an Irish-built 3D printer that uses what's called "layer-object-manufacturing", LOM. It cuts profiles out of sheets of A4 paper and laminates them together, so the raw materials are just paper, knives and a spot of pva glue, very very low cost. Reams of 80 GSM A4 paper are 50mm thick, so the layer resolution is .1mm... it's called "Mcor Matrix", if you're interested. 3D Systems released an LOM file format, .slc at the same time as .stl, it's just layers of closed polylines, you stipulate the thickness.
NetFabb, the .stl diagnostic and repair program, can produce .slc models from .stls, and animate passing through the slices, like flip-pad animations or MRIs and CT scans on TV medical shows.
Converting a curved object into a polygon object isn't very complicated, renderers do that to simplify raytracing. It gets more complicated when facets have to have defined normals so the machine knows which side is inside the volume (solid), vertices can only share x facets, and edges can only share y facets, it has to be manifold, (watertight), and there can't be duplicated facets, vertices or edges. Y'see? .stl format is deceptively simple, creating a compliant one from something else isn't.