As of Generation 6, there is another advantage to keeping your Pokemon unevolved: Bonus Exp. Points.
Any Pokemon that evolves at a certain level will receive bonus Exp. Points if its level is at least the level it evolves at. So if you have an Abra (for example) at level 16 or higher, it will receive bonus Exp. Points until you let it evolve to Kadabra. This applies as soon as the Pokemon reaches that level and afterwards gains Exp. Points, so it can sometimes happen unknowingly in Trainer battles if your Pokemon reaches the Level it evolves at, but the opponent still has Pokemon left and you have that Pokemon gain Exp. Points from them.
What this means is that starting with X/Y, if you just want to get your Pokemon to high levels, you should not let it evolve as it will gain levels faster as long as it tries to evolve, but you don't let it.
Testing indicates the multiplier is either x1.25 or x1.3.
Since there are currently no online sources to confirm this (that I'm aware of), here is the data I aquired through a personal test to confirm this.
I had a Lv.46 Vanillish with Lucky Egg and fought 3 Marchionesses with Lv.55 Audinos (Battle Chateau, Black Writ of Challenge active).
The first Audino yielded a little over 6000 Exp. Points, which brought Vanillish to Lv.47 (where it evolves). The next Audino yielded slightly more than 8000. After the battle, I canceled the evolution and fought the next Marchioness. Both Audinos gave ~8000 (The same number as before). I then let Vanillish evolve and fought the third Marchioness, this time both Audinos gave ~6000 again.