Well in that line of thinking people believed the world was flat till  they "discovered" it wasn't. People also believed that we were the only  plant in the universe, now we not only know that we aren't the only  plant. We also know we aren't the center of the universe, as was once  believed, when the first plants were discovered. A theory is just a  theory until it is either proven or dis-proven. So reincarnation is just  as plausible of a scenario as Christianity or any other unproved theory  that is out there. 
Hell Christianity isn't even as plausible because it isn't its own  religion. They had to steal other faiths and beliefs' traditions and  celebrations to make their religion more digestible to the human race.  Take for instance the celebrations Christmas and Easter. They both were  taken from pagan rituals. There are many more examples I could give here  if you would like....
One was Yule the celebration of the evergreens and the feast so everyone  could survive through the  winter.
http://en.wikipedia.org/wiki/Yule#Pr...ian_traditions
 The other being the celebration of the coming of the Goddess Eostre,  the coming of the spring and new life. This is the reason for eggs and  the bunny for Easter. 
http://en.wikipedia.org/wiki/%C4%92ostre
All organized religions today are based on the belief that their God  became human, their god died, and that their god will walk living  amongst them  again to save the faithful and pure. If that is NOT reincarnation, then  what is it? The only religion that isnt based on the Belief of  reincarnation is Paganism which most religions today have taken MANY of  its celebrations from. 
Reincarnation also doesn't mean that you will be coming back to this  world or existence as a human either. Like with the Buddhists they  believe in rebirth in all forms of sentient life. They reject the  concept of an unchanging soul like in Hinduism or Christianity. 
http://en.wikipedia.org/wiki/Buddhists#Rebirth 
http://en.wikipedia.org/wiki/Incarnation
Further evidence, if your wanting scientific proof, of reincarnation is  every time someone procreates they are reincarnating themselves.They are  imprinting their DNA and genetics into makeing a baby. 
We could go on all day about reincarnation belief. When millions of  people believe in religion. Which is another something you can't see,  touch, taste, smell, or hear. Till religion and/or reincarnation is  touchable with any of five senses it will always remain a belief and a  theory.