The only downside I foresee is being unable to search within Lightroom across shoots.
By @parkerea
While I agree with the earlier posts about how LR works best as a single catalogue, there are plenty of people who do work with job catalogues as you propose. When the job is done and portability is no longer needed, it's then common to open a master catalogue, use File > Import from Catalog to bring the job catalogue in, and then archive the job catalogue and never see it again. So you get the portability when the job is open and you have control of your entire picture collection. Managing all your pictures, not just folders (sessions) is an important aspect of Lightroom.
You also have other alternatives to moving work back and forth if you decide to import everything into a master catalogue.
One would be to use File > Export as Catalog to create a job catalogue, work on it on the other computer, and then use File > Import from Another Catalog to bring the work back into the master.
Another is to take advantage of Adobe's mobile workflow, syncing a job's photos and then accessing them on other computers using Lightroom Desktop, or even on phones and tablets. So today you might edit the shoot in Lightroom in the master catalogue, tomorrow morning on your laptop and one image on the iPad on the train home, etc, and all your edits will automatically flow back and forth. If you need to have access to full res files everywhere, you can import them into Lightroom Desktop or another of the mobile apps (obviously this uses up your space).
One last comment is that C1 sessions are more like folders, and the workflow is more like Adobe Bridge. They're not really analogous to LR's catalogues, which is why C1 has its own catalogue feature.
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