Does anybody know why they're all of the healer ethos (cleric/priest/druid)?

maybe so you would be encouraged to keep a healer around at all times?
My best guess has always been that it is for plot-purposes/difficulty
BIG PLOT SPOILER
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You loved one gets taken away by Bodhi before you face her for the final time. Since he/she probably is your cleric you have to do the vampire lair without one (if you don't have a backup cleric) which makes it significantly harder because Anomen for example may at this time be high level enough to turn Vampires and therefore make most of the battles laughably easy.
I always thought that this twist in the romance plot was how the designers kept that battle from being all too easy when Bodhi is supposed two be the evil runner-up ;)

I've beaten this game, and yet I don't ever recall having my 'romancee' removed from the party. Does this always occur, or is it only with a certain set of decisions you make?

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A romance has to go so far, and at Capter 6 I believe, they get turned into a vampire, but you have to be far into the realtionship with your NPC partner

I'm of the opinion that it's just coincidence.

It's not like the casual player will know immediately upon having an NPC join that the NPC is romanceable, so I don't agree with any line of logic that involves BioWare wanting to encourage certain NPC's to be in the party based on them being romanceable.

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The kidnap making Bodhi's lair more difficult is one of the more interesting theories I've heard, but it also doesn't hold up. There's nothing stopping you from taking more than one Cleric in the part, even more than one romanceable NPC in fact (which is exactly what I did my first game, with Jaheira and Anomen)... not to mention the fact that your NPC can be a Cleric.

first time through you won't immediately pick up NPCs because they are romanceable, but once the romance is going you are unlikely to kick them out.

You can of course have more than one cleric or be a cleric yourself but many people set for a rather diverse party to try out everything and if a class is overrepresented it's mostly tanks you have several of.

We know they had more romances planned, but I can't think there wasn't SOME element of design to the decision. Traditionally an adventuring party "needs" a cleric, but traditionally they're also kinda boring to play as main characters. So you're like to acquire one. So, you're likely to acquire a romanceable NPC.

and on the female side, they're all elves (winged, half, drow)

Good observations, ryaldin and HolyWarrior! This has actually never come to my mind but you are perfectly right. I don't have a real answer to your questions but I made one up quickly, referring to my current game with a male Kensai PC with a charisma of 4 (one point only thanks to the tome in BG1). So this guy sure has huge muscles and a quick wit (STR and INT 19; to be dualled to mage soon), but he is as charming and intriguing as an earthworm. Currently he is travelling with Jaheira, Viconia and Aerie and WOW, life can be so wonderful. Flirted at and pampered from dawn til dusk (and beyond, hi , hi), he is the center of all three beautiful elves' attention. This can only be explained with their clerical attitude (help and cure the weak, poor and suffering 'n stuff). It must be a kind of complex being enforced by their elvenkind. That's why....