Attaching the weight to your body will make you tired and ache much more quickly than will attaching the same weight to your bike. Using a backpack will also make your back sweat a lot in warm conditions. There are many ways to put weight onto your bike even if you don't have rack braze-ons, explore all of those first before thinking about using a backpack for long distance rides.
I know of two good reasons to use a backpack when cycling:
1. A lot of MTB'ers use back-packs, and they tell me that this is because, when riding in technical terrain, putting the weight onto the bike makes bike handling more difficult, and it is therefore better to have the weight on their bodies.
2. Many people who commute by bike need a bag that can be carried on and off the bike easily, and a backpack is reasonably good at this. However, I think that a messenger bag is better because it doesn't make your back so hot. Even so, I always use a pannier bag with a shoulder strap for commuting and find this to be best for me.
Neither of these reasons apply to long distance riding on the road, so I would heavily advise against using a backpack in this situation.
Last edited by Chris_W; 10-28-09 at 09:17 AM.