If I don't care how clear my wine (from kits) comes out, am I missing out on any other benefits/side effects by not using any isenglass/bentonite/sparkolloid? I've done a few batches without any additives and they've come out pretty clear (I let them sit for a month or so after stabilization).
No. The reasons for fining or filtering are:
1) Appearance. Judges (and therefore many consumers) like to see a clear product. Generally, you will do better in a competition if your wine or beer looks like what the judges expect for that style, and most times what they expect is the mass-marketed crap you can buy at Safeway.
2) Flaws or bacterial growth. If you've sent your wine to a lab and it comes back positive for brettanomyeces, then you'll want to sterile filter it so it doesn't (1) keep fermenting in the bottle and get carbonated and (2) smell like a sweaty horse.
3) Bacterial growth (part 2). I had one once where the lactic acid bacteria (MBR31, which I inoculated it with for malolactic fermentation) continued to party on long after the malic acid was gone. So I had a cloudy wine due to chains of oenococcus bacteria still thriving. I fined it with Pulviclar S from Vinquiry, which helped a little. Bumped up the SO2 also. The wine is still somewhat turbid when poured, but it's still fine, smells and tastes excellent.
Bear in mind that my answer comes from experience making Pinot Noir and Chardonnay, which I feel benefit from being unfined and unfiltered. My commercial wines are all unfined and unfiltered unless there's a problem with them during barrel aging. Though many Bordeaux and Rhone makers do the same (Ridge Vineyards comes to mind).
If your wine looks clear in the glass when you pull a barrel sample and if you can rack it from the barrel to the bottling tank carefully and subsequently settle it out, then I see no reason why it should be fined or filtered. Personally, I feel fining and filtering takes something away from the wine in order to appease critics and judges. I don't fine my commercial wines unless I absolutely have to. I've not yet (knock on wood) filtered one.