1) Yes. 2) No. 3) Maybe.
Sitemaps just help the GoogleBot do it's job more quickly. For example, if you have content that's hard to index (or parts of your site that Google doesn't index at present) a sitemap can help the bot find it's way around. It's also an efficient way to alert a search engine to new content.
It's not a requirement and are there are no easily discernible consequences to not having one, but IMHO helping Google to find your site content more quickly is a no brainer.
There are plenty of php scripts and Wordpress plugins that can generate a sitemap dynamically, so you don't have to do it manually. Once you have one of these installed you should never have to worry about it again.
Regarding RSS vs XML sitemap according to Google's Webmaster Central Blog.
No longer do you have to specify the Sitemap file type—we'll determine
the type of data you're submitting automatically. Let's take a quick
look at the kinds of Sitemap files we support as well as the ways they
can be submitted to us.
So basically you can send Google information in a number of formats and they won't bat an eye.