Considering that the library is intended to be used with the GSM shield, and that the shield is hardwired to pins 2 and 3 for the RX and TX pins, SoftwareSerial seems like the only viable option.
Yes, I know, not a smart decision, since pins 2 and 3 are not capable of SoftwareSerial on all boards.
(Actually, the library indicates differently. On a Mega, the RX pin is pin 10, which is software serial compatible.)
How are you proposing to work around the hardware issues?
Well... Say you have a GSM module which requires only TX, RX and GND. So the easiest way to connect them is to use hardware serial TX-RX with modem TX-RX. But in that case you have to use serial library and implement AT commands by your own. But this GSM library have them implemented so it is more efficient to use the same library instead of going through the AT command manual.