It’s commonplace for contracts to impose on a party a duty to take a particular action even though that action has no nexus with the one or more other parties. Here’s an example:
Each party shall pay all expenses that it incurs in connection with the transaction contemplated by this agreement.
But if you think about, Acme doesn’t really care whether Widgetco pays its expenses. (Note that I didn’t say “costs and expenses.”) Instead, all Acme is concerned about is that it won’t ever have to pay any of Widgetco’s expenses. If Widgetco stiffs one of its creditors and that creditor never looks to Acme to cover the debt, Acme would have no interest in the matter. But if the creditor does come knocking on Acme’s door, Acme would want to have some way to compel Widgetco to pay up. That would be accomplished by using is responsible for, turning the provision in question from language of obligation to language of policy:
Each party is responsible for paying all expenses that it incurs in connection with the transaction contemplated by this agreement.
Would Acme’s interests be adversely affected if instead under the contract Widgetco had a duty to pay its expenses? No. Would Widgetco’s? Again, no, because Acme couldn’t claim any damages unless a creditor were to succeed in forcing Acme to pay any of Widgetco’s expenses.
Instead, what’s at issue is your understanding. It’s important for drafters to grasp the dynamic at play in each provision. Imposing a duty instead of using is responsible for might not create a problem, but it suggests an imprecision that could bode ill in other contexts.
Obviously enough, you wouldn’t want to use is responsible for when the situation does in fact call for imposition of a duty.
None of this is novel, as I discuss is responsible for in MSCD 3.32–33. But here’s something I’ve only recently considered: Often a contract will say that a party is solely reponsible for something. I suggest that in this context, using solely constitutes rhetorical emphasis. (For more on rhetorical emphasis, search this blog using that phrase.)
For solely to add meaning, saying simply that a party is responsible for something would have to suggest that someone else, too—presumably another party—is also responsible, or at least might be. That would defeat a reader’s expectation of relevance—why go out of the way to say that a party is responsible if someone else is responsible too?
So I say keep solely out of is responsible for.