I don't feel that boxing hinders your Muay Thai skills at all. But this may have to do with the circumstances under which I learned to box.
I learned Muay Thai first. Then I learned to box. Even though I was training to actually fight as a boxer, I realized that at heart, I was a Thai boxer and was mainly learning Western Boxing to complement my Muay Thai. Therefore, I always trained a little differently, I guess.
Sure, I learned how to box correctly, using head motion and footwork and all that, but again, I kept my Muay Thai training in mind, and I modified the boxing technique to complement my Thai boxing.
Also, look at all the numbers of Thai boxers who move on to Western Boxing. Many Thais use Muay Thai as a springboard into the world of Western Boxing, because the that's where the money is really at. Samart, Samsun Eisan, Nanfah, Gansak... These were all top-notch Muay Thai fighters who also excelled in the world of Western Boxing.
As much as I hate to quote Bruce Lee (let the man rest, already!) "Take what is useful". You have to realize what it is you are trying to do, and use what will help that game. If your goal is to be a better Thai boxer, then concentrate learning the boxing skills that will help out your Muay Thai game.
I will have to say this though. This is a common mistake that I find that many people make. When I went to learn boxing, I went and learned boxing. I concentrated on absorbing EVERYTHING that I was taught with no thought as to what I was trying to get out of boxing in the long run. I didn't turn my nose up at head motion drills because of their high risk in the game of Muay Thai. I did as I was told and learned my head motions. I learned as best as I could everything that I was being taught.
Then, AFTER I had attained some degree of proficiency in these new techniques, I decided which parts I preferred vs. those I did not.
I see too many people who try and learn only parts of an art because of preconceived notions regarding what they think they need to know, or what they think will and will not work.
I have learned under three Muay Thai coaches, and as many Western Boxing coaches (at least). I have absorbed as much as I could from these instructors. Though I *personally* don't use everything that I was taught, I make a concerted effort to teach all of the techniques I've learned. Just because I don't use a technique doesn't mean another student won't.
There are certain techniques/strategies that I don't use in my own personal fighting game, whether its just something I don't like or something I just can't do well. But, that doesn't mean one of my students won't like it or be able to make it work.
I try to teach my students everything that I've learned and let them find their own way. But then, this is why Master K chose me to take over his Northern VA classes. He knows that I don't fight anything like him. But he knows that I learned to do everything HIS way first, BEFORE I made changes to the way I used techniques.
It's late, I'm babbling again. I'm going back to polishing my rocket now.