12 years as a photographer and I have never had a client tell me what camera to shoot. They hire me and trust me to use what gives the best quality for the job and within their budget.
I feel same applies for video. The GH2 is tiny, but I use a cage and SmallHD monitor for tripod shots and generally anyone is just like "wow". How the final work looks is all that matters. I don't do broadcast work, mainly corporate, DVD, web type of projects.
Realize the quality of the video from the GH2 is ahead of a lot of pro HD gear just 5 years ago, and all in a small package now.
Use the gear and be confident. If anyone ever says anything which I think is unlikely, have samples of work on your phone or an ipad, etc. Every time I show a new client a few samples of past projects on an ipad, they are ready to go, sign at the dotted line kind of thing. I've used Canons for the last 2+ years for video but now with the GH2 I have done several recent projects exclusively with it instead. I'm loving the quality from the GH2.
I do have some agency friends that ask for Canon 5D's and think they are cool for smaller, web based shows.
having said that, people should be hiring YOU for your skills and the camera shouldn't matter. Also, you can rent whatever you want whenever you want and to be associated with or tied down to one camera is pretty ridiculous.
In L.A, if you peruse Craigslist or Mandy, you'll usually see people who are looking for DSLR shooters write '5D', '7D', or just 'Canon'. It's just the way it's been. But if someone is contacting you directly because they like your reel or past work, there should be no reason why they'd tell you what to shoot with on smaller projects. On bigger projects, there's no reason for you to argue them down to your GH2 from their rented Alexa or RED.
I've had a few people ask me about my camera and all of them seem to think the 7D is a Magic camera and the only thing worth shooting on, until I show them footage of course. Dealing with Rappers and low budget music videos in the Bay area is the pits. We have a rental facility with a 20X20X12 ft Hard Cyc and I've seen so many people come thru with 7D's and even 5D's and have no Idea what they are doing and absolutely no concept of lighting. I've even had a national artist do a video at our place and the crew came in with 5 camera's 1 5D, 2 7D's, a sony camcorder and a a JVC camcorder and they all swarmed the artist like Bee's and they only had 2 yellow work lights. They were doing green screen with a Porsche and they aslo used the white cyc, they didn't want to rent our lights. Needless to say the footage came out crappy. Another funny thing is even though everyone seems to know about the "Magic 7D" almost none of them know anything about lenses and this includes those who own the cameras, they just use the kit lens for everything. I've shot with canon's and had great results but I own 2 GH2's and I'm very happy with my choice.
You need to inform your clients that they don't know what they are talking about politely. The GH2 is the Pound for Pound King of video right now. The 5D does not compare and I own one that is getting dusty.
I think the OP is probably talking about freelance gigs for other directors/ producers/ crew. Not for direct clients who normally shouldn't know one from the other. Except for the occasional one who knows a little and thinks they know a lot.... those are the worst..