It is not even a general rule. A criterion is general rule if it is generally-accepted. A criterion about which people
disagree, as in this case, isn't a general rule.
I agree that it would be tacky for someone, such as yourself, to do it, since you yourself feel it would be tacky. Again, it's a matter of judging someone by their standards, on matters where people disagree.
Hallmark is in business to sell greeting cards. The industry has made up myriad occasions that suddenly need greeting cards that did not need such in the past. A single industry's vested financial interest is not a firm basis on which to set social standards.
Indeed. I think several of us noted that inconsistency in the OP, that the thread seemed to be about cursive, but the OP seemed to indicate that the absence of cursive meant that letters therefore could not be written. However, since it was the OP, I don't think we can say that it is off-topic; rather, we can just note that the thread subject doesn't seem to match the OP. I'm not sure it really serves much purpose, though, to discuss the discussion to that depth, though.
Yes, this is critical imho: The point of writing, despite Big Cuddly Bear's insinuation, is to express thoughts and feelings. Therefore, respect for the reader dictates that writers use the means of communication that is best at expressing thoughts and feelings. Legible print clearly wins, over illegible cursive. Indeed, I feel that, in most cases, legible print wins, over legible cursive, since in most cases people can print more legibly than they can write cursive.