I wouldn't say destroying ISIS is Russia's main objective; rather, their main objective would be securing their interests and influence in the region. If ISIS gets in the way, they want to get rid of it, sure; but the same is true for any other "moderate" faction (to the extent those might exist, if at all), as well as any other foreign forces.
Precaution against what, exactly? It's not like Russia is looking to attack Turkey proper - they certainly are attacking Turkish interests in Syria, but it should be obvious by now Turkey et al. aren't going to get their way there (to the detriment of everyone else).