I mean, you can try it, if you really want to know. The thing is, you mention that "once their ttar counter has been removed", but if you've only got one Pokemon that can deal with ttar on a team, you're gonna lost a lot of games. He's just too good. If you've beaten all their ttar counters, then why would you want to lower your speed? It just gives Pokemon that would otherwise roll over and die (if you were DDing) a chance to smack you. Regirock is definitely not used commonly. That doesn't mean it's bad, but it's just not as good as the commonly used mons. There are a lot of Pokemon that are still viable, but aren't used very much, due to being too niche, or being outclassed by someone else.