I haven’t actually rejected anything yet . Rhino is out in front at the moment and setting the Grid to 10mu as above seems to a possible get-around. I am about to have a look at Grasshopper.
Sorry, let me reset the question: which ones do you consider to be toys, and why?
For this reason I use “Scale 1D” and just scale the length of a curve, or the distance between two control points. If there is a dimension already assigned to both ends, it will follow the changes of the geometry. Sort of working the reverse way compared to the dimensions in Solidworks.
Have you tried FreeCAD? It is free parametric CAD package, but it is a little bit difficult to understand the philosophy of command. Sometimes this package works like Solidworks or Inventor, but sometimes not. Till now I didn’t find how the boolean operation works compared to Solidworks (according to the task I use Rhino or Solidworks to have the best from both of the world), sometimes the constraint behave strange, but it has spreadsheet to get table driven dimension.
Coming back to Rhino: I mostly use it for mechanical design. I haven’t learned Grasshopper yet but I think it’s one of the slickest things I’ve seen in CAD.
My approach to getting “reactive” feedback in locating things is that I have the coordinate readouts at the lower left set to 4 decimal places in inches (my preferred dimension system). Then when I need to precisely locate a point, line end, etc I just start dragging it until I get to approximately where it wants to be by watching the coordinate readout and then just zoom in with the mouse wheel keeping the cursor as close to the desired location as possible. Rhino allows you to zoom in so that an inch of cursor movement amounts to about a ten thousandth of an inch location change so the dimensional readout changes in only the 4th decimal place with lots of cursor movement. I click for acceptance and mouse zoom back out. It isn’t parametric but it gets the job done and is very quick and easy. BTW: you can zoom in even further than that but my models are more than accurate enough at a tenth.
Try fooling around with this idea and I think you’ll discover it’s not bad. Another point: this doesn’t work in perspective view. Also it works better if you move in only one principal direction at a time in the appropriate view with Ortho on.
Another technique I like to use for getting precise dimensions is to just draw a helper line by a combination of typing in the start and end points or snapping as appropriate. Then I have accurate locations available for further snapping. Sometimes I’ll just place a temporary point the same way.
Not as great as Catia, but a lot cheaper and it works for me.
My point is: Rhino can be used quite successfully for mechanical design.
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Another important point: As you are building a model by creating objects that connect together, it’s essentially mandatory that you locate the points they are made from by snapping wherever possible. This ensures that points that belong together are in the same location (I think exactly, but maybe only well within tolerance). This means that when you try to join things together it will actually work.
I use the “BoxEdit” panel to make size changes quickly and accurately. I don;t know if this helps you but I use it regularly. Box on left is 5x5x10 and I copied and adjusted in “BoxEdit” to 3x5x10. You can scale and reposition also. Might be useful for you!
DesignSpark is free.
I have not used it myself, but you could give it a look to see if it will work for you.
I frequently use the SetPt command. It allows setting the x,y and/or x coordinates of “points” in either World or current CPlane coordinate systems to either keyboard input values or cursor selected position. I put “points” in quotes because SetPt can be used for individual points, for selected control points, or if an object is selected then all the control points of that object. The latter is a great way to put a line or plane at a constant x,y or z value.
I gotta agree 
but it is parametric and, therefore mush more suited to what I do.
Thanks for the hint.