TBH there are only two reasons to hold evolution off - either you just really like the base form of the Pokemon more than the evolutions and don't intend to evolve it ever, or you want it to learn stronger moves earlier/moves only it can learn but its evolution/s cannot learn. Otherwise there isn't much point, and you might as well evolve, as it will get some stat boost from evolution, etc.
Mind you evolving a Pokemon at level 15 compared to say level 50 (or even 99) will result in no difference in stats in the long run.
With that, going to close. If you have any other questions, please ask them in the Help thread sticky at the top of the section.