I don't really view this as a scam, but more for your protection. There have been at least 2 threads started about drivers being scammed for their information and instapay being used to steal their earnings from them by some 3rd party.
This is for your protection - imagine if someone got your information and was able to cash out your earnings to their own account/card that was untraceable - then you would really be crying scam!
Instapay should be viewed and used as an occasional convenience, and using it as a matter of course on a regular basis - that would scream to anyone that you are a short timer as you cannot wait until the regular payday.