The issue is the stacking benefits. Yes, on pure science totals, it is better to wait. But Planting allows you to get to the key wonders 6-10 turns faster (Printing Press - Leaning Tower/Porcelain Tower combo) and so you should do it. The bonuses from the Two Towers are immense, and if planting 1-2 Scientists make sure you cut your time on Leaning Tower of Pisa by 10 turns or greater, then you essentially return your investment (use GE on Porcelain Tower, and get free GS and extra RA power).
That's the point to planting: early game speed is highly valuable for key wonders (and on high difficulties, the pesky concept of getting to elite military techs that much faster). It you don't plant and don't get those two powerful wonders, then there's zero gain from saving.
I'm pretty sure even for "fastest" SV time the investment rewards (+25% GP, Free GS, and RA Boost) make sense. And without a doubt, if you're simply playing to 'win' and not for optimal speed records, they definitely pay off on a risk/reward basis. If the difficulty is high enough where you could lose if you don't play the early and mid-game well enough, then planting makes sense. I used to never plant, but I finally changed my ways.