It depends strictly on the user what method they want to use for a particular purpose.In this case Pakistan has chosen DSMAC,India chose to use SAR seekers.
Yes these are RF SAR seekers and not the kind of regular RF seekers used in Anti ship cruise missiles.I hope you know what SAR means.But still let me explain the rational behind this.
You see,in DSMAC,you feed the missile's guidance computer with pre reckoned images of the target and the an onboard camera on the missile takes images of the target and the guidance computer then correlates the pictures with the prefed images.
Now Nirbhay LACM also uses a similar technique that it correlates SAR imagery with the prefed target images.Only difference is,in older DSMAC method you use a camera......instead in this case we use a SAR seeker.
A SAR seeker has several key advantages over a camera like for example:
1.The efficacy of a camera or any other optical sensor is significantly degraded in cloudy or adverse weather but weather has got very little effect on SAR.
2.A camera can be fooled by using smoke,aerosaul or optical camouflage systems,but a SAR can not be fooled that way.
So you see,it's not something to laugh at.Just because you do not use it or (rather can't given the fact that Pakistan still can not develop radars or seekers on its own) doesn't mean it's a bad idea or it can not be made to work out successfully.
Several Indian companies like Datapatterns and VEM Electronics in collaboration with the LRDE has developed a range of SAR seekers for Prithvi 3,Prahaar and Shaurya NLOS BSMs,Brahmos 1 LACM variants and subsonic cruise missiles.