Riding a motorcycle, especially on a long road trip, can be fun overall but monotonous at times. Adding speakers to your helmet so that you can listen to music can be a good idea in some situations, but it does have its problems as well. Whether or not you should use helmet speakers should be based on how you react to this extra stimuli when trying to focus on the road, as well as your willingness to stay open to what's going on around you.
On one hand, having speakers that let you listen to music while wearing your helmet can help keep you alert. If you have to ensure you stay focused on the road, the music or talk can keep your mind active. It can also alleviate a lot of the boredom that often makes people drift off behind the wheel (or handlebars, in this case).
But the music and talk can be distracting, too. If you tend to be easily distracted by the radio in a car, speakers in a helmet won't do you any favors, either. Another issue is speed; if you listen to fast-paced music while out on the open road, you could find yourself speeding more than you want to if the music takes over your focus.
Using the speakers while riding could mask outside noises. This means you wouldn't hear things like sirens until the source of the siren was very close. Only get speakers that allow you to hear environmental noises, and then, do not turn the volume up so high that you drown the noises out anyway.
Also, if you turn the volume up to such a loud level, you're also potentially damaging your hearing. Speakers in a helmet are very close to your ears -- the sound waves don't have far to go before hitting your eardrums and don't have the ability to fade out much. Getting your hearing tested periodically is a good way to monitor how the speakers and your ears are getting along.
Keep in mind that although the speakers may be inside the helmet, you are not inside a car with closed windows. There is the chance that some of the sound could leak out from your helmet and create a nuisance for people around you. You may want to test various speakers with a friend to see if any sound can be heard outside the helmet.
Helmet speakers can be a good thing if chosen properly. Talk to sellers of motorcycle parts and accessories to see what they have.