One thing is patently obvious here - most sci-fi geeks do not know much about real-world weapons. Blasters and phasers give away your position? Yes, of course they do - just like modern weapons. If they didn't, fire correction would be virtually impossible.

  1. Assault weapons - all have considerable muzzle flash that pretty much instantly tells the surrounding enemy forces where you are. Muzzle flash is visible in daylight, glaringly obvious at night, and cannot practically be eliminated. "Flash suppressors" on assault rifles are there to keep the soldier from being night-blinded by his own muzzle flash. They do not hide anything from the enemy. Hopefully, you've destroyed your enemy before he can home in on you. But in real life, things rarely go so smoothly.
  2. Heavy ordnance - Tanks, artillery and other heavy guns have even more obvious muzzle flash, highly visible even on a sunny day. This is why tanks do not stay in the same position for more than a few seconds. As soon as you fire a shot, everybody in a five-kilometer radius knows where you are.
  3. Rocket-propelled weapons - missiles and rockets fly just as slowly as blaster fire and are just as obvious. It's hard to miss that stream of smoke and fire. And yes, it is possible to evade missiles and rockets. Fighter planes do it all the time, and even tanks can manage to do it if they have enough warning time. This is why most anti-tank weapons are guided all the way to the target.
  4. Tracer ammunition - as if the above tell-tales weren't bad enough, we actually use tracers to further show off our position. The problem that tracers address is one of fire correction. If you cannot see where you are shooting, you cannot correct your misses. And, contrary to what you see in action movies, most shots do not result in pyrotechnic ricochets to show you where your shot went. Also contrary to the action movie canon, you hardly ever hit on the first shot. So we use tracers. Typically machine guns are threaded with one tracer in every five rounds. When fired at night, this results in an almost continuous series of brilliant red (usually) lights shooting towards your target. From the shooter's point of view, it looks almost exactly like sci-fi weaponry. Tracers cannot be seen from directly ahead of the bullet, but they can be spotted from almost any other angle. If you want to nitpick and be geeky about it, it's entirely conceivable that future weapons inventors intentionally created beam weapons that you can see, for the same purpose that we use tracer rounds.

Note: Transitional Man's comments, apparently composed at the same time I was writing this, are all valid points as well. Hard science fiction treats these things much more reasonably. Star Wars and Trek are not meant to be realistic, but visually dramatic. Personally, I find the whole "Starfighter Paradigm" completely inane. But if you accept the space-opera setting, visible beam weapons actually make a lot of sense.