Tes, most definitely, the apothecary is an important plot element, for he helps move the action forward, yet poison is not the only means Romeo has to kill himself, so it is not essential that the aopthecary be there merely for the plot.
One possible reason is that the apothecary and Romeo share a particular quality: desperation. Since the sale of poison has been declared illegal and punishable by death, the apothecary has become hungry, and is willing to take incredible risks for the right price (as is Romeo, but his price is love).
I think the apothecary may also tie in on another level. The reason it has been deemed illegal in Mantua to sell poison is that people had been abusing it; there were so many people poisoning their enemies and former loves that it had actually become a fairly significant social problem. So in making the sale of poison illegal, the government aims to slow impulsive acts of vengence...so, the apothecary ties in perfectly to the play. Just as in so many other instances in the play, the apothecary knows there is a law against what he does but for personal reasons, he opts to ignore it.