One issue that I personally consider important in this scenario is that a leaver will irrevocably alter the pace of the fight in question. In an actual fight you tend to get a lot of experience and battles tend to be more decisive. When you're fighting a leaver, you end up stuck doing a lot of creeping and mostly cleaning bases. That tends to be slow, doesn't yield as much experience and you end up being distracted when the actual fights start. It is easy to say how to counter it, but I find it very hard to get back on track once the tense expectation of a challenging match evaporates.