A few points why:
Currently the bad deals are easy to spot since they have been listed for over 12 hours and nobody bought them :P So when looking for an upgrade you can easly skip 80% of the listed items. Very usefull if you have to go through dozens of pages.
When you loot an item with decent stats you can check AH for an estimated price by sorting buyout amount and see depending on the time remaining on the auctions what is the price where the item with those stats isn't beeing bought anymore. If you allow relisting, then all items will have over 1 day remaining and it's impossible to know if they're underpriced or overpriced and relisted every 2 hours.
When you want a specific item upgrade. Let's say your belt has 150 int 100 vit 60 AR and you want to find a good deal to upgrade it. You can do a farming run (~1 hour) and then come to AH and put those stats in the serching parameters and look for any new items that have over 1 day 10 hours left. There will be only a couple you have to check the tooltip on, you can ignore rest as you know you already looked at them. If you allow relisting then you have to check all the reslisted ones over and over :s
If sellers are too greedy or lazy to check how much their item is worth than that's their own fault imo, you shouldn't punish the buyers.