Well this is in the general discussion thread, so no need to move it to player support or whatever. Feel free to talk it out here and post your questions and opinions here.
The use of the underscore makes it easier for people to do commands on other players. For example paying, selling cars, giving weapons/drugs, etc.
It is far easier to do it with an underscore in case there is a person with the same name on the server at the same time. That is how it's been done on SAMP, but might change on LSRP:V with the addition of for example proximity commands. If there is for example two Johns on the server and you do /pay John, it'll pay the closest John to your character.
BUT! Legal Faction /forums/ have switched the format from Name_Lastname to Name Lastname. The current UCP that early supporters/staff have has the old underscore format still. But I'm sure that it's a subject to change, same goes for the in-game name and healthbar that you can see in the snippets, etc.
At the end of the day, we might keep the underscore format. I'm pulling the information out of my ass and there is two more months of server development, there will be a lot of changes and the names are not a priority compared to the other systems that need to be done and perfected.