Note that the smart LEDs only "tie up" the processor when you change their values. That's significant if you're doing some sort of animation, but not nearly so much if you're just changing colors "occasionally" (say, once per second or less.)
If you were going to send full data for a strip of LEDs over serial, the overhead would be somewhat higher than the WS2812 protocol (because serial is slower, and you'd probably have enough data to cause it to block.)
If you wanted to define serial commands like "set all to RGB value" or "Rainbow" and perhaps "shift pattern N Leds in Direction", there would potentially be significant savings, but ... you have the difficulty of pre-defining all the commmands.