The phrase "are indispensable in" is perfectly acceptable and correct in written English.
You can use it to refer to something that is necessary in some situation or process. For example, you can say: "High-quality communication skills are indispensable in any successful business venture."
So the liaison offices are indispensable in this sense".
Their firms, Goldman Sachs and JPMorgan Chase, are indispensable in raising capital for companies, creating wealth.
The Arab nations, because of their size and strategic significance, are indispensable in shaping the region's future and its security.
Taste and experience are indispensable in providing a continuous curve that will be aesthetically satisfying from every angle.
But as we see in Libya today, the United Nations and other international alliances are indispensable in providing legitimacy and reinvigorating American partnerships in times of crisis.
Power hacksaws, driven by electric motors, are indispensable in any general-purpose machine shop or tool room; they are most often used for cutting metal.
Some of the compounds of boron and aluminum, however, are indispensable in modern technology and have been widely used in many parts of the world throughout recorded history.
Indeed, you could even flip the argument to say that the deep pockets of a corporate owner are indispensable in both making good wines affordable and producing luxury wines of the pedantically fine-tuned kind.
While one person carefully removed the hot grill grate, another lifted the packs out of the coals with a stout grilling spatula and tongs -- both of which are indispensable in this setting.
"And", "but" and "so" are indispensable in linking individual sentences into a coherent passage, and they may be used to begin a sentence whenever the clauses being connected are too long or complicated to fit comfortably into a single megasentence.
Women are uniquely affected by war and their perspectives are indispensable in resolving disputes, ensuring accountability for crimes, minimising the suffering of civilians and designing long-term recovery programmes.
