The problem with weapons in space are that they put the opposing super-powers in an impossible position. Hence it forces them to make a move asap to prevent being at a total disadvantage. This is mostly because weapons in space can be used with very little warning. There is little defense, and not even any notice that allows you to prepare or setup a second strike.
Plenty of people don't think that's bad, but it seems that the top military decision makers of all super-powers do see the problem with that, and hence they have managed to keep weapons out of space.
This remains a high-level decision, so it only takes a few rational people for the game-theory to work.
Consider the external politics around mobilization. It is a big deal to mobilize externally, because it forces opposing players to also mobilize, hence moving everyone closer to conflict. The thing about weapons in space is that they are effectively always mobilized. Even if you could sustain perpetual mobilization, it wouldn't be a good strategic move.
Something similar goes for missile defense systems that are too effective. It disrupts MAD, and pushes the opposing side in to a corner. Either make a move before the defense system is up, or accept a role as a second grade power from now on. The problem, specifically, is that it removes the second-strike capability much more than that it removes the first-strike capability.
So having a missile-defense system in orbit has game-theoretical downsides both for being in space, and for removing the enemies second-strike capability.
The same does not quite hold for having the capability to launch such a missile defense system. It gives your opponent time to react when you launch it. Hence they do not need to immediately react to you having the capability. So I think research into this area, preparations, and development of the system are fine. Heck it is even important and desirable. We just need to continuously refuse to even ready such systems outside of well-announced and limited tests.