Jan and Jace are really just decoys for the greater threat, Blip.
Jan and Jace seem pretty useless in the series. They constantly get captured and used as bait for Space Ghost, but in reality they're merely decoys used to draw the villains out who would fear going for Space Ghost directly. The real threat would be to take Blip, who is infinitely more useful as a small and agile creature that combined with invisibility makes him the perfect ninja. However, the villains will constantly ignore him, believing him to be weak compared to the twins. The only reason Space Ghost keeps the twins around is so that they'll distract from Blip and draw out the villains.