Maroon on the 4th line for a year (playing up if/when needed) isn't the end of the world. He's a better fit for that role than a lot of the competition he would be facing in camp, though he doesn't have the special teams impact one would ideally want.
The thing is that the Blues need to get their talented youngsters some exposure in the NHL at some point. It's good for their development, and at some point these kids need to be tested so the Blues know what they have, and so the kids have an opportunity to build the trust/experience needed for a bigger role when then the time comes.
Assuming that Maroon gets more than a 1 year deal, the Blues will have Tarasenko, ROR, Fabbri, Schwartz, Schenn, Steen, Bozak, Perron, and Maroon locked in for multiple years...with Barbashev, Soshnikov, and Jaskin being under team control and in the mix as well.
Ignoring Thorburn and Sundqvist, that's still 12 guys. The talent of Thomas and Kyrou will demand NHL time in the near future. Kostin isn't that far behind. Sanford (and Blais) loses waiver exemption after next year, so a decision needs to be made on him quite soon as well.
I don't mind signing guys like Maroon if the intention is to ship someone out when it's time to clear some space. My concern is that the Blues will get so caught up with "competing" that they'll hesitate to do that when the time comes, favoring experience and depth over youth, and they'll ultimately suffocate/squander some of the better talent they have in their system.
Maybe that's less of a concern now that the Blues have more control over their AHL team, but I still think it's important to find some NHL time for your kids for a lot of reasons.