I used to use this with Gmail, but switched to Sparrow a few days ago. I've been interacting with my email in a much faster and more fluid way since, which wasn't possible using Gmail/Fluid.

That's definitely a valid concern.

I use fewer site-specific browsers than I otherwise would because of memory usage, but these days many of us have 4-8 GB RAM and running one or two SSBs doesn't break the bank, so to speak.