Yeah, I'm with CB.
The guy with the 3.00 ERA in 175 IP will allow around 59 earned runs in that time. The guy with the 4.35 ERA (I found a middle ground) in 230 IP will allow 112 earned runs. So, to make up the 55 IP lost by having the 175 IP pitcher, you would need your spot starter to allow less than 53 earned runs in 55 IP. He does that, then the combination of 175 IP starter and s***** spot starter is better.
Spot starter can also just be a long reliever.