For each of the cases you described there are simple and cheap work arounds.
In the first example even by using a plunge saw you will run the risk of hitting the saw horse. You can solve the problem by adding a sacrificial top to your saw horses that can be cut into and then replaced when it gets tatty. This is just glued in place with a few dabs of glue and knocked off with a hammer when it is time to be replaced. With a sacrificial top no saw will permenantly damage the saw horses.
The second example still won't require a plunge saw. If you lay the case on the floor face down, you can set a standard circular saw to cut at a specific depth. Where a plunge saw differs is that the body of the saw is mechanically supported as you sink it into the cut, and then it will usually be used with a special rail that acts as a straight edge. With a standard circular saw you have to support the saw and slowly lower it into the wood and slide it along a straight edge like a long spirit level that is clamped in place.
To quote a popular woodworking article:
"How Plunge-cut Saws Work
The Festool and Makita saws’ plunge operation is like moving your wrist in a hammering motion. The DeWalt saw moves a bit differently. It rocks forward to make a plunge cut with an action similar to moving something from one spot to another – lift, move, then set back down. Switching between the two different motions causes one to think, but individually, their use is intuitive.
What makes these saws operate as they do is the blade set with a “toe-in” design. This allows the saw to cut at the front of the blade with the back of the blade held away from the freshly cut edge. Minimal burn and little tear-out on the money side of the cut is the result. This is also the major difference between the finished cuts from these saws and those of a circular saw when used in combination with a straightedge guide.
There are other features common to these saws, such as track adjusters to dial in the exact fit for a smooth slide, a lockable arbor for quick blade changes as well as blade depth-of-cut adjustments. And each saw is set up for dust extraction via an external vacuum. (See page 53 for more comparisons.)
A significant feature found on the DeWalt and Festool saws, but not on Makita’s saw, is a riving knife. At first, you may question a riving knife’s importance because the blade retracts into the saw if the tool is lifted from the track. But even with that action, there is still an opportunity for kickback and a riving knife defends against that action."
These saws are only really useful if you need a really fine finish because there is no way to hide the cut or the time spent sanding or planing the cut edge will result in lost chargable time. Like I said, I have known professionals who have never needed to buy a plunge saw. If the tool bug has bitten you and you want one, then don't let me talk you out of it but I do think for the tasks you describe a circular saw will be just fine.
Paul.
