You're gonna get the full gamut of replies on this question, ranging from "you'll be fine" to "Don't do it unless you want to die!" Reality is that it depends on the type of tire (steel cords are tougher to plug than aramid/polyester cords), location of hole (center of tread vs near the sidewall), size of the hole, and your skill/experience with the plug kit. Honestly, if the tire has 50% life gone, I wouldn't risk plugging a tire for only a few thousand miles of riding. If it's a brand new $180 tire, I'd certainly try it. Your other option is to demount the tire and use a patch or take it to a tire center and have it professionally repaired. Most places won't repair a MC tire "OFFICIALLY" but if you simply carry it in and do it on a cash basis (no receipt) they'll do the repair.
I've done it both ways--plugs and patches--and run the tires for thousands of miles until the carcass was done and had no problems. I had one tire with steel cords in the tread that I was never able to repair without a slow leak. Try it and see.
Last edited by badjuju; 12-28-2013 at 11:04 AM.