You are right in such sense that asking a question from particular user can kill soul of forum/community and your question can remain unanswered by others are unable to see.
But there is another point of view also.
Let's suppose, there is a community/forum where all kind of users come to ask questions/ give contributions from novice to experts.
Some experts also provide professional services also. Now as user, i am looking for someone to work for me. I want to make contact to ask a question from particular user. Because that user lives in same city/area where i am, or he/she can provide solutions to my problem. I am interested in hiring that person.
I want to contact him/her so ask more question about cost etc. I want to contact him/her. @gidgoreen has answered to that situation as mentioned above.
Now comes point of implementation like how forum can be constructed to answer such situations. The ways come to my mind are like this
1. You ask question and get responses from experts and contact that person whom answer you like most and want to get their advice/opinion or simply ask for their services.
2. You see people who are most active and have expertise you are looking for and want to raise their attention to your question (like facebook's feature of mentioning someone) so that they will know by notification someone has mentioned you. However question can be viewable to everyone.
You as being expert should have ability to control who can contact you. May be people who earn some worth can ask you question.
In short, point is that some are expert who can provide service like arranging events, delivery etc so you want to contact them.