Between Fordprefect and Kingtiez the points for or against the need for lend-lease or shared vehicles has been fairly well covered but I would like to add the following (generally limited to ground battles).
Having experienced the initial Allies vs Axis matchmaking and how it impacted vehicle performance, I’d argue that with the current match making free for all, lend lease and sub tree vehicles contribute to stable gameplay, more than they detract.
Export and lend lease Shermans and T-34s are very important to the game in my opinion. At Rank II/III it provideds nearly every nation with a good medium tank workhorse to act as a benchmark without negatively impacting the nations that developed them; US still has the best variety of Shermans, Russia still has the best variety of T-34s.
Additionally, its important to remember that people who engage with the forums like this represent a very small minority. Most players won’t ever complete a tech tree, even fewer will grind out several tech trees to completion. Having variety within single tech trees is critical to ensuring that new players and more casual players aren’t hamstrung by limited variety or overspecialised vehicles.
A final point to consider also is that as vehicles become more modern they tend to become more alike.
You essentially end up with the ‘western MBT’ and the ‘eastern mbt’. This is to say that the auto loading russian/chinese MBTs are more alike than they are different. I’d argue that the western MBTs with 120mm smooth bores in big turrets with composite cheeks are also more similar than they are different. Its the same for the Wheeled light tanks and the SPAA.