The only real downside that I would see to this type of setup would be the massive hit to battery life. While phone manufacturers have made great strides to reduce the power footprint of wifi on devices, it's still nowhere near as efficient as the phone's CDMA radio for interfacing with the cellular network.
If I use a SIP client on my Samsung Epic as a basis for comparison, the talk time is 50% or less using wifi than making a standard call through Sprint's CDMA network.
I get better battery life with just data only wifi offloading I use y-5 to turn on wifi when I'm at the office or home and I get at least 50% more battery life out of my phone with that setup. and I set my wifi to never sleep. Much better life than relying on 3G for background and foreground data services.
I don't do it for my cap, I'm on grandfathered unlimited on Verizon, just for battery life. I think it depends on the phone and setup they may have implemented it well.