Is it normal to frame a knee wall without a top plate?

Build it like your buddy says. I did my bonus room over the garage a few years back and started out with the same plan as yours, just running studs up to the rafters. Tore half of it out and redid it with a proper plate on top and I'm glad I did.

First reason is drywall. Rafters run on their spacing, which almost never lines up with where the wall meets the ceiling, so with no plate you've got a seam with nothing solid behind it. You'll be hunting for backing with scraps or deadwood, and in a few years that joint is exactly where you'll get a crack. With a plate there, the board edge lands on wood every time. Second reason is the wall itself goes togather way better. You can build it flat on the deck, square and plumb, then tip it up and shim to the rafters. Toe-nailing every stud up against rafter framing individually means the wall is only as straight as the rafters are, and garage rafters are ussually not that straight. Now your load question, and this is the part that actually matters. You're not overthinking it. A short wall like this can absolutely pick up weight from the rafters as they flex under snow or shingle work, and if that bottom plate is sitting mid-span between joists, that load has nowhere good to go. Over time you get a soft spot or a creak right in the middle of the floor. The fix is cheap: nail solid blocking between the floor joists under that wall before you set the plate, or shift the wall so it lands on a joist if you can. Either way the load gets a path down. So yeah, doubled plate cut to the pitch is not overkill, it's maybe an extra hour of cutting. And the blocking under the bottom plate is the piece your buddy probably didn't mention.

Another thing I learned the hard way on a similar knee wall: skipping the plate makes every stud its own little puzzle. Mine all got cut and shimmed to whatever each rafter was doing, so they weren't even close to uniform, and later when I went to hang shelves and a workbench rail, nothing lined up like it shold. A plate gives you one clean reference line across the whole wall. The pitch cuts sound intimidating but they go fast once you set your saw angle and just batch-cut them all at once.

Not sold on the 'build it flat and tip it up' part of that first reply, at least not for a wall like this. If the top plate is cut on the pitch, the angle has to line up with the rafters dead on, and once you're shimming a tipped wall to a set of crooked garage rafters you've undone the whole benefit of a clean reference line anyway. I built mine in place, one stud at a time, plumb off the floor and scribed to the rafter. Slower, sure, but each stud acutally touched wood instead of sitting on a shim wedge that creeps out over a winter or two.