As the out-going Editor of the website - and a long-standing member of the RHS (which I kept up during the eight years I worked for the RHS) I thought I'd add my two-penneth-worth.
The running costs of My Garden are tiny, as very little of the on-going web costs are tied up in the forums. Those members of RHS staff whose jobs are under threat need to be aware of this as it will have no effect on their positions.
The blogs are a different kettle of fish as the RHS does pay for some of the external bloggers because it is seen that their contributions are a vital part of maintaining the interest in and content of the site. But they're not paid a fortune by any means!
The forums are a vital part of the website and, as the RHS is a charity whose remit is to improve gardening and horticulture and bring it to a wider audience, it is felt that the forums must have as wide an audience as possible - and therefore should be free to all. I totally agree.
As an RHS member and an ex member of staff, I have no qualms about the forums being free to use and accessible to all. I hope it stays that way, because it's the only way to build up an online community of gardeners and those keen on gardening - which is, after all, what we all are.
There are never any problems in gardening - just opportunities!