I also have recently become somewhat addicted to 1-pocket, and, IMO, the potential gains from playing the game far outweigh any potential losses.
I remember several years ago, when I was playing a game of 1-p about once every 6 months, that the game was very frustrating to me because I allowed all of the ducking and defensive maneuvers to get me out of stroke, which led to me missing balls I felt like I should have made. I mistakenly thought that the imprecise aspects of the game (which is how I thought of the the defensive parts) were having an adverse effect on the things that required precision (which is how I thought of shotmaking/ball running). At that point in time, I would probably have agreed with the local player who said 1-p will not help your 9-ball game.
What I finally figured out, though, is that the defensive aspects of the game, if you approach them properly, often require more precision than the shotmaking. If you think about it, banking a ball from your opponent's side into a particular portion of the stack, to move balls your way, while sending whitey two rails to freeze to the backside of another ball near your opp's hole, which is not an uncommon shot in 1-p, requires much more precision than most of the offensive shots you will take on. When I began to approach the game in this manner, by envisioning a very precise result for my defensive shots, and attempting achieve that result, I found that I was able to stay in stroke throughout a 1-p session, and that my success with offensive shots improved dramatically.
My advice is to stick with the 1-p, but to also spend a good portion of your time playing the rotation games, because, as other posters have stated, there are many shots you will need in 9-ball that are almost non-existent in 1-p.