It really, really depends on the volume of work you do, and how you want the final garment to look. There's other ways to finish off the edges that "work" just fine, but they are more time consuming and don't give the same look as a binding attachment. If you go to a reputable (and usually an indie) sewing machine dealer and tell them precisely what you want and politely ask for a binding demo, they will be able to show you and you can decide for yourself.
For me, I'm really REALLY wanting a coverhem machine... I used to not do enough knits to warrant it, but it's kind of getting silly for me at this point. I just have to wait til we move and have more room...
If I were in your shoes, having a coverstitch would be essential:
For stretch garments with elastic edges (like legs and arm holes), the coverstitch function alone can create a nice 'double stitch' over a tuned edge with elastic--it's fairly common to see this used. (To get this look you need to overlock elastic to the edge first, then turn that edge under before using the coverstitch). Your overlock's stitch on the reverse side will bind off the fabric hem edges for a nice look.
Using the overlock alone, without elastic, makes a great turned edge, and this hem can be very narrow, so it just fits the overlock's sewn stitches--this could be used for necklines, armholes, and hems on skirts, pants and leggings. The edge of the hem will be covered by your stitching.
Bound edges: If you cut your own binding from the swimsuit fabric, you can create matching bound edges around necklines and armholes. This binding can be simply cut cross grain in the correct width (you may need to play with this binding to find the right width, since it stretches and becomes narrower when applied).
Yes, it's pricey, but this machine will probably pay for itself--especially if you branch out to sew fashion knits too.
good luck with your knit projects!