I don't for a moment think that all business is evil but I do think many lack impartiality and, having worked in jobs where advertising was a large part of revenue, I can see how business can be allowed to lean on impartiality. I'd be very worried about there being similar pressures involved in any sponsorship.
I don't think central government is perfect but generally they are several hands removed from deciding what museums should be displaying/what exhibitions are suitable/the general tone of the musuem - most of the money is distributed by quangos and so on that have guidelines rather than straightforward commercial gain to use as a yardstick. I'm not against corporate sponsors being brought in for special exhibitions - say, the Media Museum were to run an exhibition on animation sponsored by a big animation company or film distributors - but if the whole museum were to be run by Dreamworks, would there be the same breadth of content?
Thanks for the explanation about where the 10% figure came from BeckandCall.