Nothing stops you from giving your Sergeants regular Chainswords either. We obviously want more!
(also if one day Eviscerators and Powerfists get different profiles again it would be weird for them to have Eviscerators instead)
They already have different profiles. Eviscerators are AP4, power fists AP3.
Ah right, my bad. Though AP-4 is usually overkill compared to AP-3 anyway.