On behalf of those of us here that aren't tradesmen, if someone comes up with a reasonable excuse that the missus will believe eg "you might need a dropsaw for that job" then we're going to run with that....
Try to be supportive, use phrases like "yeah a drop saw is good, but you might also need a triton workbench to mount it on", or constructive suggestions like "don't buy brand x, they are crap..." even "you might have difficult using a drop saw to do a mitre join, perhaps buy a router as well".
Just saying....
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Finally.. Someone read my post.
Every big job I do I buy a "necessary" tool. (Sometimes I actually use it on that job)
My wife is happy for me to do it as it does make the jobs much easier with the right gear.
You could use the square and hand held saw, but that means marking where the cut is needed, then measuring back along the board to allow for the saw base. Easier to by a drop saw, that had a wide enough cut, or if you can get away with it a sliding compound saw for even wider cuts.
On the joints, mortise and tenon, and dovetail are mostly furniture joints
Mitre joints as described and a lot of other joints like scribed corners for cornices are made to hide shrinkage, things not being square etc. (and are legitimate joints)