As awesome as that would be (seriously, imagine no browser testing), it's just not feasible for most web companies.
(1) Sometimes the client's website gets so much traffic from older browsers that not adding something like jReject is irresponsible on our part. We can't give the majority of their visitors a poor user experience.
(2) A lot of people work for companies whose entire computer system is locked, and they're not allowed to download and use a separate browser than the company provides. Unfortunately companies like this don't want to pay to update all those computers, so their employees are hobbling along with IE 7 or (eek!) even IE 6. Should they suffer because their company doesn't want to upgrade?
In the long run, it's not that difficult for us to add something like jReject, and it boosts people's experience of our websites. Sounds pretty worthwhile.
