IMHO, as long as another country has ballistic missiles, a development of ABM is not only necessary, but essential.
Yes, I acknowledged that it is necessary to have an ABM umbrella... but as mentioned in my previous post, China might not be in the position for this system yet.
Mainly because... at present moment, it really is questionable of how effective the ABM is. The system that China is building cover only its own ground... and I questioned the numbers of ABM missiles China has or is afforded to build. And how many of these missiles will be required to knock off one ballistic missile even from... say, India in a real war situation when there are huge number of unknowns, such as location of launch, which area will the missile hit, the terrain, the environment, the weather, etc.
And my arguement as to why US is in a position to develope and deploy ABM, mainly because, she had huge number of allies all over the world... allies such as Japan, South Korea, Philipines, Taiwan... even Thailand, etc. And I believe that if US was to request for placing her ABM in Singapore, the country will not resist the idea too.
So China or any of asia country who happen to launch a ballistic missile (in the most unlikely scenario) at US, their missile will be knock off sky without even leaving the airspace.
However China do not have such a luxury. When the missile was launched, the limited range of her umbrella will only be able to hit and destroy a ballistic missile only when it is near her airspace. And if the missile was to escape her ABM umbrella, there simply is very little or nothing China could do to knock the missile off.
US on the other hand could deploy and set many layers of ABM, even shorter range SM-3 and land based patriot all over her allies, thus a ballistic missile really have to escape larger number of defence the US had.
Also US had a very powerful second strike capability (nuclear submarine, destroyers and carriers), thus even if the ballistic missiles of enemy hit her mainland, she could still retaliate with catashropic effect.
China do not have such ability. In recent years we saw alot of developement in submarine technology for China, but up till now, she only have very limited second strike capability.
Thus I would believe that resources should be allocated more for larger and more capable second strike capability such as more nuclear submarines (more of them), destroyers (more of them), aircraft carriers (at least 3 or 4).
This would be a more viable defence, in the sense that no country would try to attack mainland china, as they knew, the country would have a credible second strike even when the mainland was hit.
I believe this is much more important than building defensive ABM umbrella that had questionable capability and efficiency.