This seems like the key post talking about this issue:
I think I agree with that for [quote], because [quote] affects the entire block of text; it’s nonsensical to start a quote in the middle of a non-quote line.
But there’s nothing terrible about that for [spoiler]. An exception should be made for inline multi-line spoilers.
There already appears to be some special case code. Consider these examples:
[spoiler]spoiler text[/spoiler]
[quote]quote text[/quote]
These behave totally differently. The spoiler works, but the quote doesn’t.
spoiler text
P.S. I don’t really care about [quote] but I think that last [quote] example should work, too.