If the runner has the breaker for the ice after Marker, and they're paying 1 to break 1, you've paid the install cost of Marker to increase the cost to get through the server by 1. That's roughly equal, but probably not worth a deck slot.
If Marker is in front of a...
- Barrier, it is redundant. The barrier already ends the run, so it doesn't really change anything. If the runner has a fracter, they can get through, and if they don't, they can't.
- Code Gate, and the runner doesn't have a decoder, it might help stop the runner from proceeding. It is best if the code gate stings upon face checking it, although that list is short (Crick, Upayoga, maybe Aiki or Harvester...). Even if the runner jacks out after encountering Marker, it has done its job.
- Sentry, and the runner doesn't have a decoder or a killer, you have done a great thing. Also, why didn't they jack out? Also, why were they running against without a sentry breaker? Maybe you just used Midori to swap out a boring Vanilla with an unrezzed Komainu. Oh! Or maybe they let the Marker sub fire, and then they approach a Cobra, and then you rez your Marcus Batty and win the psi game and trash their killer and then you get to fire the Cobra so you trash another program which is probably Corroder and then deal two net damage and then the ETR keeps them out and you can score!! Blammo!!!!
Yeah, so uses are limited. I seriously think the Batty thing is probably the best idea, and that's a 3-card combo with positional ice and a psi game, so...good luck with that.
P.P.S. Yog breaks for free ah balls don't play marker