An RFP sent to ten or more law firms generates lengthy and detailed responses. If firms can reply as they see fit, your evaluation team will have a hard time picking out of the mass the comparable answers. Better, much better, to tell firms precisely how you want them to present their responses.
I have seen this done with spreadsheets. Every answer sought in the RFP had its place in the spreadsheet. The rigidity of that approach is apparent, but it certainly made the collation of answers dramatically easier. A less Prussian approach asks for the firms to complete tables in Word documents. The law department can easily copy and paste tables into Excel. At a minimum, instruct firms to follow the format of the RFP questions when they respond.