People would just substitute religion for some other lame excuse (say, being a fan of a different soccer team) to keep killing each other...
See, I don't think this is true. Certainly eliminating religion would not end all suffering or war or hatred completely (you'd have to be an idiot to think that, really) but it would do a lot of good. Firstly, it would remove a lot of homophobia from the world. Some people would still be bigots, mainly on the grounds of finding it icky, but I think a large majority of people would no longer oppose gay rights. Secondly, the Israeli/Palestinian conflict would become a step closer to being resolved, although of course years of built up hatred on both sides would still need to be overcome. The same goes for various other conflicts of an entirely or partly religious nature, e.g. Northern Ireland. Thirdly, the Middle East would improve immesurably, as religious tensions that existed between now non-existant sects dissolved overnight, and religiously based oppression (against women, free exchange of ideas etc) would be severely weakened (although I think coming to the point of equal rights for all and a belief in freedom of speech would take longer, just because old habits die hard). Fourthly, the cause of scientific thinking would be helped (particularly over there in America), as, for example, no-one would now want to block the teaching of evolution in schools or stem cell research or future advances in technology which religion might get hung up on for no good reason.
In terms of harm, it's hard to say. The churches often act as organisations for charitable collection and distribution, or as community centres, so losing them without a secular replacement would be bad. I don't think producing a secular replacement would be so hard though. A lot of people might occasionally feel a bit more lonely without someone to pray to/chant at/sacrifice goats to, and a few people might feel a LOT more lonely, possibly to the point of suicide (I don't think studies show atheists are more likely to commit suicide, but it's possible that those who would be unable to cope without a God merely never become atheists in the first place). Maybe some drug/alcohol addicts find it harder to kick the habit without Jesus to pray to in moments of weakness (OTOH, Mel Gibson is hardly a shining example of the all conquering abilities of prayer to keep you sober). I'm fairly sure morality in general would be unaffected in most cases, and improve in some, without religion. On the whole, I think losing religion would be a big plus.
Another thought, RE:The idea that people would kill each other for some other reason without religion, do people who advance this idea really think that 9/11 would have happened without religion? Or the witch burnings/Spanish Inquisition (I bet no-one expected that!)? Or the Crusades, which were a specific attempt to convert the "Heathens"? 'Cause I find it really hard to emphasise with that point of view.
Of course, this all assumes that it's even possible to get rid of religion. In fact, some new religions would just take the place of the old ones. Hopefully they'd be more in line with a modern moral/ethical view, but still, they'd be there.
