I just switched to a heavier bicycle (8 speed Shimano Nexus IGH) from an 80's steel racing bike (Schwinn Tempo) for my commute. My commute takes about 4 minutes longer over 3 miles, but the ride is much nicer. It floats over chipseal and rough pavement in a way that the Tempo could never do. My commute is 3.2 miles one way, mostly flat with a very small hill towards the end. I can definitely feel the difference in speed and responsiveness, but it doesn't bother me. And now I can really strip down the Tempo because I don't have to try and make it be both a commuter and a weekend-fast-fun machine.

Bottom line: if the bike works for you and you're happy with it, it's all good. If you want to get another bike, get another one. No further need for justification.