Without optical isolation of your ignition circuit from your receiver circuit, any interference that your ignition makes is going to go right to your receiver. Yes, 2.4ghz systems are much better at ignoring that interference than 72mhz systems were, which is a big reason why gassers have gained so much popularity in the last 10 years. But ignitions can still play havoc with your radio gear, so it's just not smart to run both systems from the same power source without an optical isolator.