Western Boxing is definitely an art in its own right and a great one too. It's ergonomic, economic, pure and simple and gets the job done very well.
It complements any other art perfectly, as mentioned by many; to cover the essential in SD, boxing itself needs some complementing. Grappling, weapons and groundwork need to be covered as well, to a minimum. The movement, angles, distances and timing technique in boxing leave nothing to be desired, not to mention the conditioning part. Good common sense is required, though, with the constant head contact, to avoid too much exposure to that.
The karate style I practice has kihon practice in hook punching ("mawashizuki"), use of the higher stance and cover for mid-distance work ("jodangamae") in addition to the usual, wider and lower karate footwork and shielding ("chudangamae" or "hanmigamae") and striking of more like the competition karate persuasion. Full contact sparring is highly recommended, if only to get the feel of it.
Normal sparring in the style is non-competition-like, sensible contact stuff with no prohibited technique: it usually ends in the ground, low kicking and striking to all distances is prominent. Hardening of striking areas and learning proper contact when striking is taught; makiwaras, heavy bags and focus mitts are used extensively. Very few people get enthusiastic about real tameshiwari, though - breaking things isn't obligatory.
I took a while of regular boxing practice at a local club to learn from the specialists and still benefit from the experience, both MA- and SD-wise. My trainer was approaching 70 at the time, I was 23, with seven years of good training in store and 20 kilos bigger than him. I never scored a single good hit on him, and Lord knows I tried. But, to highlight the risks involved, he got rapidly demented and ended up with a stroke all in one winter season's downhill. Too many hits in the head during sixty years in the gym.
Every now and then I still do some German-style school boxing and light sparring to see to that the eyes remain open when being attacked. It definitely works and I recommend at least this approach to any more or less "traditional" martial artist. No style is complete without the experience of full contact and learning to be... well... somewhat comfortable with it.