I've experimented with a couple different multi-monitor setups. Notably, dual 24" displays, and a single Thunderbolt 27". For a while I had Skyteam Platinum and shipped my Thunderbolt display as my free extra checked bag on a couple trips to Brazil and Holland. When the shipping was free and I was staying in the same place for a while, it was marginally worth it bringing the second monitor. When I was making more frequent jumps between cities it was definitely not.
Personally, I find having additional monitors to not be a huge productivity boost to my work (software engineering, team/account management, writing, coaching, and managing an import/ecommerce business) relative to a good virtual desktop setup, particularly when weighed against increased pack weight and shipping costs. The place where it helps me the most is to be able to have API docs open alongside my editor/IDE, but I can do the same on retina macbook display, so having a lightweight and portable setup wins out for me.