Gero thinks Vegeta entering the battle will make things more interesting. The status quo is Vegeta > Piccolo, Super Saiyan is intended to be a complete surprise. Vegeta's superiority over Piccolo should go unquestioned, unless gives direct evidence of the contrary, Vegeta is the superior ally to Piccolo, he's the big dog, not little Piccolo. There's no reason to make Piccolo bigger. If Piccolo can make a difference in regards to Gero vs Vegeta, it's only because of Gero's miscalculations, as well as Piccolo making up a far greater percentage of Vegeta's power than most fans think.
I have Suppressed Piccolo at a hefty 7.5% of Super Saiyan Vegeta's power. If Gero's estimations on Vegeta are off as suggested, we can see such a turnaround being significant enough to change the tides.