It does work in 10.3.4., if you have the US standard keyboard selected
Press the Option and the E keys simultaneously, then release them and press i. You'll get í. áéíóú ÁÉÍÓÚ. <--- I just typed all that using the US keyboard in 10.3.4 with the Option E combination.
I happen to use a custom software keyboard layout resource I created myself to type all so-called “special” characters much faster. If you do that a lot, you should consider using the Spanish, Italian or French keyboard layouts that come with OS X (built in).