In most cases, you're right. But if you have a very expensive non-MTH loco, it might be worth it. Also, in my opinion the 1990's Weavers, made by Samhongsa l I believe, are superior runners to the Chinese recent MTHs. Then there's sentiment......
Sentiment led me to upgrade a 1950 Lionel #736 Berkshire and a 1951 Lionel 624 switcher, installing new motors. The 624 runs well, but with weakened megne-tracyion isn't a good puller. PS2 in the 736 disclosed that it wouldn't run well at slow speeds, for mechanical reasons.
