The half stop faster lens was a big deal when f/1.4 was the fastest SLR lens you could buy. Conversely, the 55 f/1.8 was significantly cheaper than the faster lens, but could use the same filters (because the slightly longer, slightly slower lens had the same barrel diameter).

A couple years later, you could collect a $100 premium for f/1.2 vs. f/1.4 (which was real money in 1970).