With Mac Pro, is software RAID 0 worth it?
Yes, RAID 0 is faster than using the drives independently of each other. Whether or not its noticeably faster depends on what kind of application you're looking at. Some setups benefit hugely from a RAID 0 setup, while for others, its pretty marginal.

There's also the convenience of everything being treated like one big drive.

I will however say that spreading your data like this has one huge disadvantage. You've multiplied the number of potential points of failure by increasing the number of drives in your array, and the data isn't being protected by any sort of mirroring or large scale redundancy. When a RAID 0 array fails, it fails hard, and you'd be well served by having a very robust backup scheme.

For the record, my Mac Pro's drive are not in a RAID configuration.