The trackball is definitely a pricey piece of kit to buy and never use.
But if the goal is to allow for 3-4 players at parties, etc. then just make sure you're being brutally honest with yourself about what games you really see being played in such situations. If it really is the 90's beat-em-ups, then go for it. Personally, I find those games dull as hell to play today because you just keep mindlessly mashing buttons and pumping in "credits" every 8 seconds to continue, and if the amount of times I've seen those games get played for more than 2 minutes straight is any indication, then most of my friends seem to agree. To each their own on that front, I guess.
In my case, even if I did have 4 sticks rather than the conventional 2, their frequency of use would be a fraction of what the trackball sees in 3-4 player situations. Trackball games like the various bowling titles, Shuuz, Shuffleshot, and Golden Tee are perennial crowd-pleasers at my place, presumably because a) casual- to non-gamers find them approachable, meaning the whole crowd is engaged rather than just the gamer geeks; and b) it opens up the gates for friendly smack-talking and friendly competition (incidentally, I've yet to see somebody say "HA! My TMNT score was 87 to your pathetic 45! WHO'S YOUR DADDY?")
And that's just the party game situation--there are also the classic trackball games that some would consider to be mandatory, such as Centipede, Millipede, Missile Command, and a handful of lesser-known but nonetheless great titles like Reactor, Wacko, and Marble Madness.
Aside from the already mentioned "secondary" benefit of the trackball as a "mouse", it can also be used as a general purpose analog controller. While nothing like the real deal, trust me when I say playing Pole Position, Star Wars, Major Havoc, Tac/Scan and countless others can actually be pretty fun with a trackball; not so with a joystick.
As for the "add on" track ball idea, I suppose that's a fair compromise, but I would think a trackball that is off to the side of a big wide 4-player panel would suck due to being way too far off center--I could be wrong there, as I've never actually tried it.
I'm probably sounding like a slimy trackball salesman here, and that's really not my intent--I'm just trying to give you my perspective and experiences to help you make the most informed choice possible.