Please don't sing while your conducting. I think conductors who specialize in choral music are more guilty of this than others. The singing conductor, however, is unfortunately not exclusively limited to choir directors. Many instrumentalists know the odd and disconcerting feeling when a conductor begins to hum along with a line. This is full on crazy behavior.
If you are singing, you are not listening.
If you are singing, you are trying to do it for them instead of allowing them to learn to do it.
If it is more meaningful for you to sing while the ensemble is rehearsing, you should probably join a chorus instead of standing up in front of musicians and waving your arms around
It is perfectly acceptable to demonstrate something by singing when the ensemble is not actually making music. Most of the time, musicians can communicate more information by actually making music than by speaking. I can often more demonstrate a phrase shape more quickly by singing or playing it than I can by talking about it. I also like doing this because it involves less talking.
Just don't do it while they are practicing. Another way to think about it is in the same context of shouting instructions to your ensemble while they are practicing. It's just like kindergarten, really. If you want people to respect you and listen to what you say when you talk, you should be quiet when it's their turn to talk.