I don't think its important at all to put together a set from just one company. Having said that, the only non-Titleist clubs in my bag are an Odyssey putter and Cleveland wedge. This wasn't due to anything besides liking the clubs more than the others I tested them against.
My irons and woods are slightly outdated at the moment. I would imagine that pitting them against any new clubs, they would get beat my most of the brands out there. However, when I do choose to buy clubs I always test out as many as I can and just go with what feels best. So if you like the look of a club, go try it out and compare it to any others you think might compete.