Good question Rellims I have actually enjoyed trying out themes with just the weather spot, since I couldn't imagine doing so before when I was hooked on MB. For the themes with the OCD slot I throw in Ubertwitter
Even though I do have MB I try to hide it as far away as possible so I find myself using themes with just one icon spot (but I believe Bryan and I would be in the minority on this). To answer your question I put either Opera or media in that slot.
well, it really depends what theme you are using and what apps you use the most... i tend to hide my meterberry sometimes... and with a hidden dock theme i will either put bbm or opera mini up there. if it isnt a hidden dock, it depends on the number of icons i have on my homescreen but ubertwitter or opera