Say you had 10 15K SAS drives in an R10 set. At a pretty common expectation of about 180 IOs/sec/drive, you'd expect to peak out at about 1300 IO/s. With 7.2K NL-SAS drives, you can expect more like 80 IO/sec/drive, and 490 IO/s out of that 10 disk set. Or, 37% of the performance.
It gets even worse in RAID parity schemes where writes have a higher impact.
Also, not that NL-SAS != SATA - there are features in NL-SAS that make it faster than SATA.
So yes, there are plenty of people doing just fine on NL-SAS and SATA class drives, you just have to be aware of what you workload is and make sure you stay within that envelope.