I think the situation where this quotation is most commonly used today is rather the obverse of Portia's plea. She was asking for mercy but making the rather obvious point that she could not force the plantiff to grant it. (strained meaning forced). The usual context today, I believe, is to refute another persons claim to have behaved mercifully or generously by pointing out that they actually had no choice other than to do as they did. Example: "Well, I gave two thousand pounds to charity last year."
"Only because your accountant told you to pay it to avoid surtax. TQOMINS."

I think the situation where this quotation is most commonly used today is rather the obverse of Portia's plea. She was asking for mercy but making the rather obvious point that she could not force the plantiff to grant it. (Strained here meaning forced). The usual context today, I believe, is to refute another persons claim to have behaved mercifully or generously by pointing out that they actually had no choice other than to do as they did. Example:

Well, I gave two thousand pounds to charity last year.
Only because your accountant told you to pay it to avoid surtax. TQOMINS.