I'd say it can be worthwhile for certain situations. For example, if you click on 'Support IrfanView' at:
http://irfanview.com/You'll see how donation payments are setup for him. In these types of situations the vendor can earn significant revenue, but consider that his program is free, incredibly popular, fully featured, and highly appreciated by its audience. These factors certainly help drive a lot more donation volume than the typical freeware app with a donation link. At the same time, it doesn't take much effort to setup a donation link, and you will only know what results from it after you've taken the time to try it. So we do encourage vendors to try out offering a donation link and quickly learning how lucrative it appears to or not to be. I can tell you through experience that if you have a free app out there with factors similar to the factors described above, there's no reason you can't earn thousands per month in donations. It depends to what degree you hit those factors, such as your degree of popularity and buzz-worthy fan base.
At the same, I wouldn't rule out charging for the game and seeing how that goes. When I first put out Morpheus in 2001, my partner didn't think anyone would pay a single dollar for the freeware program. We listed it for $9.99 and got some sales. Then we tried $29.95 and $49.95. After testing pricing and also trial expiration scenarios, adding a couple of relevant cross-sells in the cart, we were able to optimize the downloads and today our average order size is over $65.00 on a product we gave out for free and then for a long time charged no more than $10 on. So testing, tweaking, and optimizing is worth it.
Dan