Depending on how much I am carrying and whether the bike allows it, I actually sometimes would prefer using front panniers only. I did the PCH with front panniers and only the tent on the rear rack.

Also I will mention that as has been said they can easily come off when not on tour. In fact many front racks will need to come off to carry the bike on many car racks.

For lower cost rack there are plenty on the market that are inexpensive and work fine. I like the Blackburn EX-1 and some of the Axiom models for the back and the Nashbar low rider for the front. Those can all be found quite cheaply and I have used them happily for a lot of miles of touring.