I think that veganism has the strongest moral argument and vegetarianism in general, assuming it includes large quantities of say, eggs, isn't the best thought out moral argument unless it's a compromise. However, that being said your points 1, 2 and 3 are pretty good.
Drawing a "bright" line is helpful for promoting vegetarianism. When someone says to me one day in the future "So you haven't eaten meat once for years" I want to reply "Not once, and I've never missed it or felt I've needed it and I feel great." A response which might be good in making vegetarianism seem like something that they should try. But, if I reply, ""Well, that was this one time when there was a small piece of ham in a pasta salad and I just thought sod it, and this one time my son had leftover chicken and it was going to be thrown away anyway..." then it is going to make vegetarianism look weaker. So, that is part of my reason for drawing a very firm line on meat, fish, and eggs.
I think you will find vegetarians staying away from the moral debates precisely because their moral position that allows them to eat eggs and dairy isn't as well thought out, and I think vegetarians are more likely to have other reasons too, health, ickiness of eating flesh, and so on. Where it's a moral position my suspicion is that a vegetarian's moral position is more simplistic rather than the result of thorough research, i.e. don't like thinking of eating cute animals, wouldn't want to kill an animal themselves. Whereas vegans have a well thought out moral position that they are rightly confident in, and know they won't lose the argument, and therefore more likely to jump into the debate.
One argument in favour of vegetarianism rather than veganism is that the intensity of suffering for animals being killed is much higher than the intensity of suffering for dairy products, since cows are large so less animals badly treated and killed per unit food, and also cows often have a relatively better life than pigs or chicken. However, for this argument to work, you really have to stop eating eggs even if you continue to eat dairy.
In my first month of being mostly vegan, when I was basically still in a transition, I went on holiday and most eating places didn't have any vegan options except something really boring like constantly eating chips or salad. I decided no to seafood, but yes to cheese options at the time (both of which were plentiful), but I had to admit, it didn't make a lot of sense. That was my last month of eating cheese. Since then I ate cheese once when it was the only food available and I had a sod it moment, and once when I ordered a pizza without cheese and they sent it out with cheese.
Even veganism, even a strict veganism, is about harm reduction rather than elimination, although a strict veganism may eliminate all deliberate, direct, conscious harm. However, even a vegan can be, say, in favour of something like building a new factory to produce mobile phones, even though it might say require destruction of a forest to build the factory which may cause many animals to die. So in that sense it's possible to argue that we are all drawing the line between morality and convenience or our own desires to some extent.
However, in 100 years it's possible that eating cheese occassionally in social situations might become perceived quite differently, the kind of thing that would be frowned upon by most. Could happen, attitudes change.
Last edited by Jamie in Chile; 11-02-2016 at 02:06 PM.