Bidirectional redirects are good user experience
I would go ahead and implement the redirects, it seems like good user experience to me. I suspect that it isn't always done because:
- Mobile sites work on desktops even if they aren't the ideal experience.
As opposed to the other way around where a desktop site is often nearly unusable on mobile.
- Your redirect rules will probably need to be kept up to date in two places. If you have separated URLs and code bases for mobile and desktop you will probably have duplicated lists of user agents between the two.
- There may be some user agents that are ambiguous. You may not have any data on whether some user agents would prefer the mobile or the desktop versions and maybe you shouldn't redirect those either way. In other words your rules for redirects from mobile to desktop may not be just the negation of your rules for redirecting from desktop to mobile.
Bidirectional redirects are OK for SEO
From an SEO standpoint, Google is fine with bidirectional mobile redirects. They specifically say so in their separate mobile URLs documentation:
Different websites implement different redirection policies. Some websites only redirect mobile users visiting a desktop page to the mobile page (“unidirectional” redirects), and some websites redirect both mobile and desktop users if they visit pages on, respectively, the desktop and mobile sites (“bidirectional” redirects).
For Googlebot, we do not have any preference and recommend that webmasters consider their users when deciding on their redirection policy. The most important thing is to serve correct and consistent redirects, i.e. redirect to the equivalent content on the desktop or mobile site. If your configuration is wrong, some users may not be able to see your content at all.
Also, we suggest giving users a way to override the redirect policy, i.e. allowing mobile users to view the desktop page and allowing desktop users to see the mobile page if they so choose.
This WebmasterWorld forum thread has several rewrite based implementation suggestions for bi-directional mobile redirects.