I'm going to partially disagree with some of the opinions here. When it comes to speakers, quality does matter. Yes, even for arcade gaming.
Now if we are talking 70's/80's era arcade games (i.e. 8 bit and earlier sound) with the typical bleeps and bloops of that era, speaker quality is less important. Cheap speakers are going to sound the same as expensive ones. But when you get into the 16-bit era, speaker quality can make a difference and this is where a 2.1 system comes in.
FWIW, I use Promedia's on my main PC and I've used various other cheaper 2.1's for arcades. Currently I have a cheap pair of Logitech's on my arcade test unit. And I will tell you there is a significant difference in sound between the two, even for arcade games. The Promedia's have clean, punchy bass and the satellites properly reproduce the mid-high range sounds. OTOH, the Logitech subwoofer sounds muddy and boomy, and does double duty with the lower mid-range frequencies. The resulting sound is pretty crap, imho.
Now if sound isn't that important (and I've found for most people it isn't), then a cheap 2.1 system will suffice. But if you want better sound, then it's worth it to spend a little more and get a set that is half-decent. And especially so if your arcade is used for modern PC/console games as well.