Whether or not a wing absolutely needs to be glassed depends upon the structure. In a properly-designed wing, plywood joiners will do as good a job supporting the wing as will a glassed joint. Some models, such as many Joe Bridi designs had no plywood center joiner, but used wide fiberglass tape that was epoxied down. That's how they were designed, and it was an easy job to join the wing panels.

I've had large models with foam wings that had spars added and heavy plywood across the center section.

Whether glass or wood, a properly-designed and properly-constructed model will not come apart when it's flown within its intended flight envelope.

If you feel more secure by glassing a wing that has a plywood joiner, then do so. Your confidence in your model goes a long way towards having fun with it.