It's lazy and tiresome and generally a way of stringing out storylines long beyond their natural span. It's the point where a writer has been backed into a corner by giving one of the characters information which they would not keep to themselves but, for dramatic purposes, cannot be revealed to the audience. And so it seems that, instead of coming up with a creative way around, the old "Well, why don't we just give them amnesia" card gets pulled out of the pack.
It fills me a kind of all-pervading weariness whenever this particular chestnut gets pulled out. It doesn't raise my ire in the same way that the "it was all a dream" ending does (I'll never regain that time I spent watching Boxing Helena*) but it comes a close second. Basically, it's a way ofmarking off narrative time until you can get the characters to the point they need to be at in the plot. I never think "oh goody, a nice juicy amnesia storyline". No, it's the inward groan, the rolling of the eyes and the gritting of the teeth til it's all over.
Just to point out, in now way does this mean that I'm not enjoying rewatching Lost cause I certainly am (with the exception of any of the Boone and Shannon episodes - snore). It just struck as one of those things that I hate in otherwise engaging stories. Anyway, that's my bit for today, as you were, at ease.
* Oh, did I spoil the end of that film for you? Good because now you won't be tempted to watch and I've just saved you both valuable time and a sense of frustration at being treated like an idiot. You're welcome.