…There are, however, those who use Solo as an exploit (for want of a better term). I have read comments from commanders in the forums that basically come down to, "I was in Open. Got into a fight. Started losing. No worries. Log off. Log into Solo. You can't find me now!" It's possibly players who do this that are tainting the reputation of legitimate Solo players.
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And what exactly would be the difference in outcome if the player log out and goes into the kitchen to make a tea or coffee, then reads a bit in the forum makes a comment about Solo vs. Open and after 30 minutes logs in again? The other player wouldn't know the difference.
What if a CMDR is in a fight, starts losing, jumps to the next system, drops into normal space, waits a few minutes and then jumps to an other system. The other CMDR will probably never find the first CMDR. And that is not considering instancing.
Yesterday I had a fight with an other CMDR (obviously in Open Mode and btw: 07 to that CMDR) after the fight I went to the station and logged off - why? Because it was late and I had to go to bed. Did I use an "exploit" because I probably refused that CMDR a revenge?
Short: the problem in your example isn't mode switching. The problem is some strange idea of "honor". Fight goes wrong, CMDR flees - end of story in my opinion. What that CMDR does after logging out is completely irrelevant.
And since you mentioned "legitimate Solo players" - what exactly is that? Mode switching turns all players into legitimate Solo, private Group and Open Mode players. Or in other words: There is not such thing as a legit Solo/Group/Open player - there are only players.
(Just to be sure, this is not about combat logging, that is an other problem not related to mode switching)