Odessa is worth far more to Ukraine than it is to Russia.
Russia has already seized control of everything worth seizing by taking Crimea. Consider the below map of hydrocarbon deposits in the Black Sea:
Now consider this map of what is going to happen to the exclusive economic zones (EEZ) of Russia and Ukraine after the Crimean annexation:
Ukraine may keep a couple of gas fields that it will undoubtedly be unable to exploit without Russian approval or participation. But it will lose the big ones. Skifska and Forosa together will by themselves produce the oil equivalent of 142,000 barrels per day.
In other words, annexing Odessa only gets Russia access to Transnistria. And Transnistria is useful only as a lawless zone; it has no strategic value as a constituent part of the Russian Federation.
On the other hand, Ukraine needs Odessa desperately. Why? Well, it is nice to have a port, but as a general principal western Ukraine does not depend on seaborne traffic, save for one key import ... oil. Ukraine’s big remaining refinery is there and it is the logical location for any future LNG import terminals. Ukraine has already been nose-punched by Lukoil’s decision to cut off crude shipments; losing all coastal access will be bad.
In the longer run, say five years, the flow in the existing pipelines from Russia can be reversed to import gas from European LNG terminals. New refineries can be built and the country can continue to buy refined products from Lukoil. But the short-run would be bad.
The thing is, taking Odessa would not be easy. Too much effort to hurt Kiev in a way that does not help Moscow. I doubt that the Russian government is going to do it.
Absent serious internecine fighting, I would bet against a Russian invasion of the east at all, let along a drive on Odessa. Right now, it is looking that Ukraine might federalize. It is only a short step from that to a peaceful divorce for the east, likely leaving the key coastal areas in a smaller Ukraine. Even without a de jure divorce, federalization will give Russia most of what it wants. Much easier than a war.
But I am not one for pretending that I have seen into Vladimir Putin’s soul. I could be wrong. This story is a little worrisome. Fighting in the east could force Putin’s hand, given the reputational capital that he has put on the line.
I understand why the Ukrainians are doing it and I would certainly order the same were I in charge in Kiev ... but at some cold-blooded level it is not rational. Ukraine has more to lose than Russia has to gain.