There seems to be a stigma about couples who don't tie the knot at a KH, but, I've known a few who were in good standing but chose to hold it elsewhere for a variety of reasons. One reason was convenience. There are a few spots in my town where the reception hall also has nice space to conduct the wedding ceremony. It's great when you only have to walk a few yards to another large room that's well designed for feeding a large crown, contains a dancefloor with nice acoustics and has in-house catering. It's a one-stop shop and I love it, especially when I'm in a wedding party because we don't have to mess with coordinating a commute or make guests wait while we have our photos taken.
Another reason I've heard that has merit is, the couple wants a place that's photogenic. I've been to weddings where very good-standing JWs took their vows in parks, gardens, beaches, near waterfalls or inside historical landmarks with interior beauty. I mean, let's face it--who wants their pictures taken in a drab room with industrial carpet, low-lying suspended grid ceilings, fabric conference chairs, flourescent lighting, vinyl wallpaper, a skinny, handmade podium, microphone wires and a tacky poster with the year's text in bold font letters in English and Spanish hanging off to the side? No amount of floral designs or chair rearrangements can make that look any better.
Despite that, anytime I've attended a JW wedding off site like that, I hear murmuring about it. Usually among the elderly or parent-age generation. Sometimes, the murmuring happens at the event! But it usually takes a day or two to surface as car group fodder. The following week is when all the pioneer crones and retired sheriff elders start running their mouth about "how bad it looks not to marry in a KH." "One of them must have done something wrong and cannot qualify for a KH wedding." blah-blah-blah...
IF you marry in a KH, all the bridesmaids and groomsmen are subject to extensive background investigations to prove their worthiness to participate in the ceremony. If held offsite, the bride and groom can select whoever they want to be in their wedding and the elders can say or do nothing. I see no advantage to marrying in a KH. None. And I feel pity for any EXJW couple who married inside a KH and have a permanent reminder of their former lives forever sealed in their wedding photos. How sad.
Bottom line: Witness or not, don't do it! It's worth dealing with people's gossip and rumors.