If all the other proofs of the relationship are solid, you don't need these letters but every little bit of evidence does help.
I think the need for the letters is dependent on the type of application (inland vs. outland) and the region the application is coming from.
In our region specific form, for our outlansd application they were very clear that you had to include 2 statuatory declarations:
You can see below form the Appendix A Doccument Checklist - Immigrant.
5. PROOF OF RELATIONSHIP TO SPONSOR
If you are a common-law or conjugal partner, also provide;
• evidence that your relationship is genuine and continuing,
• has existed for at least 12 months prior to your application,
• details of the history of your relationship,
• at least two statutory declarations from individuals with personal knowledge of your
relationship supporting your claim that the relationship is genuine and continuing,• any other evidence of your relationship and communication.
Yuri - I suggest reading over your application very carefully to see if this is required, and if so how many.