Although I commented a bit in the discussion, I was hampered by being personally involved , feeling down and I don't think I managed to articulate exactly what I was trying to say. I've had some time to think now, and I want to try another stab at it.
What I was trying to say was that readthroughs are a collaborative acting experience. As with most collaborations, to enjoy them you want to feel involved and that you are making a difference to the outcome. That what you do or don't do will make a difference to the how well the readthrough goes. Clearly major roles guarantee involvement (and preparation for them can take you through periods in which you are uninvolved). However, they are not necessary for involvement - even small, unimportant roles can make a difference, depending on what they are - depending on how what difference your acting them well or not makes.
It will not surprise you to learn that Buffy contains a lot of random vampires, whose destiny is to have a fight with Buffy & Co, and to disappear in a puff of dust. Their job is to show how Buffy is the punching, kicking, slaying kind of girl that she is - to run on, look menacingly at the protagonists, and lie down dead when Buffy shouts 'stake'. Action scenes are hard enough to pull off if rehearsed, turning into a confusion of bodies without unifying purpose. Unrehearsed they achieve, at their best, mediocrity. So playing them is not hugely rewarding - you can play them well, you can play them badly; you could be replaced by another person, by the narrator, by a ventriloquist's dummy, by a sack of flour - it doesn't really matter. It'll have no effect on whether people think that the episode went well or badly. I played a lot of such vampires.
In contrast, one of the roles I played was a teacher playing hangman with the class. The teacher had only had about three lines which were completely irrelevant to the plot. Nonetheless you could use those those could establish a character and give context to the rest of the episode. With this role I felt that how I played it made a difference and contributed to the readthrough going well.
In the Buffy readthrough, the people casting parts took pains to ensure that people generally had something to do each episode. Looking at what I ended up doing, in half the episodes I played vampires (or vampire analogues) and ended up feeling relatively uninvolved, quarter of the episodes I played teachers (or teacher analogues) and felt I'd done something little but useful, the remaining quarter of the episodes I played sensible parts . Because I was feeling ill having lots of episodes in which I didn't need to worry about anything much was a good thing, but if I'd felt better, I think I'd have been less sanguine about it.
When Emperor was soliciting comments about future Buffy readthroughs, I commented that I agreed with his suggestion for reducing the number of participants because "if I'd been feeling better, then having sensible roles in only four of the twenty-odd episodes would have chaffed". It is true that resolving the issues in this comment would deal with the underlying problem - if you're doing more sensible roles, you're uninvolved less, and having fewer people helps ensure there are fewer episodes in which you are playing only vampires or vampire analogues. However, it is still a misanalysis of my underlying issues, so I thought I'd create this post to clarify the issue. 
(I am well aware of the conflicts between this conclusion, and the fact that, if the same conclusion would have been reached for the previous Buffy readthrough, I'd not have allowed to attend - something that I would have greatly regretted. But it is what I think.)
 If you follow atreic's criteria that there are five main characters, then I don't think that I was a major character in any episode. But while five main characters is true for Shakespeare, I think that the ensemble and ongoing nature of Buffy means it typically has about 10 main characters. Using this this expanded definition, I was a major character four times (against a 'fair' expectation of about eleven).
 Maybe I should count Pete as two major characters. Playing Pete was fun.
 The sentence at the end of the post, finishing "whinging is unappealing" was a particularly divisive rhetorical device, casting people who wanted to disagree, and (in my eyes) the person who made the original comment in the part of the whingers. 
 Of course, this post is doing very similar things by providing criticism, not appropriately explaining the whole circumstances, and then playing the 'you shouldn't attack me' card of depression. I should evidently take the plank out of my own eye ;).
 In my own mind more than in anyone else's, I suspect.