Some do use a dovetail. Truss rod's are not necessary if the neck is thick enough and has support wood or carbon added. The main advantage of the truss rod is that it allows small adjustments to the relief of the neck. It's a really personal feel, there is only so much a truss can do.
It does change the action somewhat, but that is not the purpose. One thing that a truss rod can do on a classical is add weight to the neck. In some cases it can be minimal, but I do think on inexpensive import guitars with truss rods, they are heavy and way overbuilt.
I can tell you there are times when I wish I could just turn a truss rod to make adjustments. A spanish heel neck does change the relief from time to time with humidity changes...it's perceptible but not huge. Still it would be nice to have the luxury of making a seasonal tweak.
As guitars get later in age, they can benefit from fretboard planing and new frets, if there is enough wood to take off then a new fingerboard is often the solution.
In a classical guitar the Spanish heel is a practical and very stable neck for the duration of ownership, that would be you...So I wouldn't worry too much. If you do feel like you would like to have the option to adjust relief every so often, then it's a good option to add if you have a builder make you a personal guitar.
Purists won't like the truss rod. But it's not a bad thing. If you want to see forward progress on neck ergonomics look at Matsuda guitars
Recessed, bolt on, elevated disconnected entirely from the soundboard