I used to switch between US and DK layouts, it's just so much easier to just have one layout. In my opinion, learning US Intl is worth it. Besides, it's pretty predictable for Spanish characters: á = altgr+a, é = altgr+e and so on.
Yeah it's great for Spanish. But I found that knowing (and using) native layout is more valuable to me because when I need to type on someone else's keyboard I'm not lost and can still touch type. If you never need to do this US international is indeed superior layout.
Trying to remember more than two layouts becomes confusing very fast. AZERTY and QWERZ are the two worst ones, fortunately I never need to use those.
I have a strange ISO/ANSI US keyboard, with an ANSI left shift and an ISO enter key.