Right.
You're right in that you won't be required to submit a letter from your employer - however, self-employed sponsors usually wind up needing to supply MORE evidence, not less. You'll need to include tax returns (to prove income and show you're paying tax on what you make), P60s, etc. as well as the usual bank statements. As you're not drawing a regular salary from an employer, you'll need to prove without a doubt that your income is sufficient to support your wife without accessing public funds, AND that this income isn't going to change drastically in the foreseeable future. You might even be asked to provide evidence of contracts/account agreements for past projects.
Unfair, I know, as it's not as if a traditionally-employed sponsor couldn't just up and quit his or her job immediately following a visa approval, but it's just the way things are.
ETA: There also seems to be a direct corollary between how much you have in savings and how closely the ECO looks at your employment documentation - so if you have significant amounts of savings, you may be able to omit some of this paperwork.