The same goes for newer planes like the Liberty, sure it's newer, but is it actually a better trainer?
Well, every aircraft design is a compromise but as time goes on, improvements are being made to certain aspects of the design that actually do improve an aircraft, without too much downsides, when it comes to operating an aircraft.
Examples:
Nosewheel vs. tailwheel made aircraft much easier to handle, for novice pilots, on tarmac (and to some extent even on grass).
Fuel injection made engines a little more efficient and reliable, and did away with the carb heat knob
FADEC made engines even more efficient and reliable, and did away with the mixture knob
Advanced in aerodynamics greatly diminished traits such as adverse yaw, made airframes less draggy but therefore harder to slow down before landing
An invention called flaps meant that at a whim you can greatly increase the drag of an airframe so sideslipping was no longer needed
Better understanding of stalls, and stall warners made spinning far less likely, so spin training could be removed from the syllabus
But... The GA fleet currently consists of everything from the 30+ year old stuff to the very modern. The question is: what are you going to train for? If you eventually end up flying a Diamond TDI, or even end up flying a bus, then why would you need to know about carb heat, mixture, side slipping, spinning and such? But if you eventually decide to plunge into the other end of GA, flying vintage aeroplanes and/or aerobatics, you definitely do.
At the end of it all, what probably influences things the most is what the students intentions are. If his intentions are to fly for the airlines, by all means let him do his PPL in a Liberty or Diamond, with nosewheel, glass cockpit, FADEC controlled engines and all adverse aerodynamics tweaked out. In contrast, if the student intends to fly for fun, then have him fly more "traditional" types during the PPL, perhaps including some Super Cub or similar flights once or twice.