So why are these Vivillion so important that I feel the need to give up my hard-earned legendaries like Articuno, Zapdos, and Moltres? Because they weren't hard earned. In fact, I put no effort into catching these legends as they weren't even mine to begin with! I got them from trades. As someone who likes the breeding mechanic in the game, I often end up breeding WAY more pokémon than I could ever need (which is also way I offered Unburden Treecko and Iron Fist Chimchar). Well, rather than release them, I often Wonder Trade them. However, if I get a particularly good spare, I put it on the GTS and ask for an easily obtained legendary pokémon. Typically, the best thing to ask for are pokémon not used competitively, so pokémon like Thundurus or Landorus don't really work. You may have to wait a while, but eventually, a competitive player who doesn't care about their Articuno or Moltres will trade you. So here's the progression, I start off with a bred pokémon of which I have many. I trade it for a pokémon that is one per game. I then trade that one per game legendary for a CURRENTLY widely available event pokémon.
So again, how is this an investment? Well, this particular event only lasts one month (July 7th 2014 - July 31st 2014). That means people who have or get X and/or Y during July will get one. Unfortunately, someone who doesn't have them will miss out. Same goes for someone who skips X and Y and gets OmegaRuby and AlphaSapphire in November. At that point, I will have leverage because I have MANY Fancy Vivillion. I have done this in the past with other events as well. I would rent a spare copy of a game and get as many events as I could tolerate (download, trade to my game, reset rental copy, repeat). With the collapse of places like Blockbuster, though, it really isn't feasible anymore (unless you have a Gamefly account I guess). Anyway, those people who missed out are going to want the events they missed, in which case then I'll be in business. I'll be able to get version exclusive legendaries (I have yet to get a Kyogre I'm happy with), mega stones, and all manner of shiny.
Of course, my goals aren't all about me. I do run into those poor sorts that never get to go to events, have never caught a shiny, etc. I have been known to trade away events easy to people like this so they don't lose heart. I know I've thrown around a lot of Wishmaker Jirachi to kids who didn't start Pokémon until Diamond and Pearl. I'm also quite certain I'll meet some newbies who have just started playing pokémon with OmegaRuby and AlphaSapphire in November. I don't think it's fair they would have to accept never getting the Fancy Vivillion since they didn't have games at the time, so I'll likely hook them up. I've also used my spare events to use a prizes for tournaments, which gives people who missed out on events another opportunity to get them. In the end, I think helping others out is probably most rewarding part of this investment, so if you get the chance to get some spares and pass them on I'd say do it. Even if the Fancy Vivillion never get so rare as to get you any shiny you want, you may end up brightening up someone's day.