To what end? What would be achieved with it?
What target will be affected and what strategic or tactical calculus will swing in Pakistani favor?
Even if it did, what is the time to impact that will allow those MIRVs to have any meaningful dispersion?
It will be a waste of time, money and technology which is better invested in Ababeel and Shaheen.
Even the BMD argument makes no sense because:
1) India so far plans to deploy a BMD system in Delhi and Mumbai only and
might plan on expanding the project's scope. It is reserved for very high value targets, not for battlefield use (think Fatah-1 vs Shaheen)
2) BMD systems, even America's THAAD, have difficulty intercepting missiles. Shaheen-III is already nearly impossible to intercept because it constantly changes its trajectory. This method is more cost effective than MIRV, and could be installed on Ghaznavi - then again, for such a short range missile, why go through the headache? Might as well fire two missiles.
3) What tiny warheads do you plan to fit on them?
4) If you want to expand the missile's size, payload, thrust etc. to accommodate larger MIRVs, you are making a whole new missile. Why not invest in an SLBM, SLCM, or other project.