Marking a custom field as required is not enough. There is already an open FR you can see it/vote it here (http://thwack.solarwinds.com/ideas/1892) to increases the power of Custom fields. Yes you can say it is mandatory at ticket creation time but I may not know the information at this time but if its part of a workflow or approval process I may find the information out at a certain time then have a different cab group use that information further down the line.
It really feels at this time custom fields are a feather that was tacked on but they hold very little useful power.