You say you don't really need an important subjob for the first 30 levels. You are wrong, I can tell you that people that are building up parties will be going for a WAR/MNK or a MNK/WAR or a THF/WAR over a WAR/RDM; it actually happened a couple of days ago for a party I was building up. I asked everyone what they thought of a RDM/WAR and they said no deal and that they'd leave if I invited him.

So in conclusion, sure you can try it out, but you will get no party invites and find it hard to get to level 30. Also a underleveled subjob, even during the levels of 20-30 is a bad idea and also gets looked at when people are deciding who to invite.

You don't solo for long, and you will eventually need a party to get ANYwhere, so I wouldn't go for some crazy job combination for levels upto 30.

Lionx is right.