Is it really necessary to treat expressions of gratitude like plague and pestilence on this site?
Assuming they are really treated like plague: necessary? no; useful? yes.
When having a problem I seek good answers that help. Votes are indicators and there's no need to duplicate their functionality. I find sole "expressions of gratitude by readers directed at contributors" just noise. I welcome comments that maybe say "thank you" and add something useful (e.g. point out why the answer is better than other answers). I don't think these get deleted mindlessly.
One needs 50 reputation to comment but only 15 to vote up. If a reader can comment to express their gratitude then certainly they can cast an upvote (or/and accept the answer, if they are the original poster).
Few times I received a "thank you" comment without any upvote (or formal acceptance). I strongly prefer the other way around.
[expressions of gratitude] get weeded out in no time.
At least some of them disappear after they served their purpose. I have flagged comments that say "thanks" under my answers (not necessarily here, I contribute mostly to Super User). If a comment says "thank you" to me and I've read it, then it's no longer needed at all. I've also edited few answers that mentioned me (e.g. "Thanks to Kamil's helpful comments I was able to solve…").
Even if such expressions of gratitude got deleted without my intervention, I wouldn't mind. In my opinion the more technical U&L SE is, the better. I would hate it as a social network. I want to gain knowledge, share knowledge, solve problems. I really don't care who is grateful to whom, especially if I'm not the addressee. I care if questions are interesting and if answers are helpful.
In any case of expressed gratitude all readers but one are not the addressee. Why should hundreds or thousands (or hundreds of thousands) of readers have their time wasted by presenting them "thank you" addressed to someone else?
Is this really considerate?
Yes. Considerate to these hundreds.