That's all true enough, but there are a number of sliding scales in there, as well as some objective factors. For example, a rebuilt and somewhat upgraded (thermals and ERA, maybe a new engine and trannie, possibly Drozhd or Arena or an equivalent if those can be supported by the platform..._and_ pan out) T-34/85 or M-4 (or Isherman see: http://en.wikipedia.org/wiki/M50_Super_Sherman) is much more likely to be supportable by a Third World army than are any first line western, or Russian, for that matter, tanks. And likely to be about as useful. Some armies couldn't, of course, while others - on the margins, say - perhaps could. Even if they could maintain the more modern ones, though, the costs are often prohibitive. I'm not sure of the costs of running and maintaining an M-1, today. I do recall that it was over 50 dollars a mile, for Class IX alone...about 25 years ago.
Then there's the sliding factor of the enemy. A truly crappy enemy allows less skill and less innate instinct - likewise a less capable vehicle - than a truly good one. An irregular enemy, without heavy weapons, permits more than a regular one, with. (Though if that regular enemy's tank fleet is all deadlined...)
Roads, too, are an objective factor. The best tank crews in the world aren't worth much without a fleet of trucks and roads to supply them over, along with logisticians and maintenance types capable of managing them. Difficulty of maintaining them, too, counts, as well as societal discipline to keep the crews doing their part in that.
In short, it's a very complex, case by case, question.
Bookmarks