I think they have merit for consideration. I personally didn't stick those in my speculation pool for a couple reasons. Marrow Shards and Mutagenic Growth are very specific meta cards, so they see a bunch of play and then they disappear again forever, and I'm not personally able to buy/sell enough of them fast enough to even bother targeting them. If you have a quick/easy outlet, or you deal mostly on MTGO, I could see that being a little more reasonable. Mental Misstep is still $2, even though it's only legal in standard. If you think they'll unban it in modern, then that's a speculation worth making, otherwise, i really don't see it moving to much.