The summoning effect can alter the RS gold level of your combat, therefore your companion should be able to fight alongside your character, regardless of single-fight restrictions. The bonuses that you get from your creature more than make up this loss is simply not true mathematically.
Ex: Steel titan gives 20% defense boost. 20% of 99 equals 19.8 This is equivalent to 20. 20 defense levels would increase combat levels, which could boost it five levels (defense*4=1cb). 99 summoning grants you 12 combat levels. What are the OSRS buy gold 7 other combat levels? This seems a little cheesy. This creature is clearly part you. You created it and are the one who controls it. You also are the one who can destroy. Why isn't anyone changing this?