OK, then I'm not sure what your point at all is. I'm pretty sure you were referring to Sinclair in the earlier message ("To my mind, the characters of Sheridan and Sinclair are as important as each other. ") What if O'Hare came by the sets lots of times during season 2 but never showed up on camera? That would still fulfill this condition of returning to the sets, but it wouldn't mean much for the content of the show. Also, if we are talking of O'Hare himself -- he continued supporting Babylon 5 for the remainder of the show, making convention appearances and always talking positively of the show.
My point is, Sinclair appeared on the show in season 2 and season 3 so we got Sinclair twice after season one. O'Hare helped promote the show for all five seasons. Zathras was very happy in the end.
Sinclair and Sheridan are as important as each other in terms of contribution to the Babylon 5 universe. That's an aside however. Given this however, the gravity of Sinclair's role in the grand scheme of things certainly wouldn't preclude O'Hare from returning on a semi-regular basis, especially considering the importance of Minbar and the Rangers in the coming seasons.
Coupled with this, we have O'Hare's continued efforts in helping promote the show, which again, ordinarily, wouldn't disenfranchise him from appearing on the show.
Despite the above however, O'Hare returned to the show only once after wrapping filming at the backend of the first season.
The point is simple, given Joe's comments around the 'change in direction', and the desire to move certain strands of the story from Sinclair to Sheridan, the impression given is that there's no animosity between Joe and O'Hare, which is reflected in his continued support for the show. This, together with MOH's sterling efforts on behalf of promoting the show, and the continued gravity of the Sinclair character to the central narrative, left me, as a viewer, slightly disappointed that O'Hare only returned to the show once. Hardly a great reward for his efforts. Now, admittedly, part of this may have been that MOH didn't particularly want to return, but given his support of the show, I find that unlikely.