I can pretty much guarantee that the public transport company's announcement was written by an attorney whose objective was pretty much exactly as porsche described.

I'm an attorney, and I frequently use this phrase in written communications with opposing parties. The main reason is that if the communication comes up as evidence in front of a jury, it makes me sound a little bit less like a jerk.

I say: "Please be advised that if you fail to vacate the default judgment entered against my client, I will advise him regarding his options, which may include the commencement of a civil action against you."

I mean: "Vacate the default judgment entered against my client immediately or he'll sue your sorry *ss and I'll twist the knife. If you don't, and he does, and I do, the but-you didn't-warn-me-defense will not be available to you, and I'll just giggle maniacally as your blood rolls into the gutter."

The former sounds at least a bit less demanding and coercive, though, doesn't it?

I avoid, however, using it in communications with "aligned" parties (clients, co-counsel, etc.), for the reason to which legal alluded: I don't want to confuse them -- it's not their blood I'm looking to spill.