You are still going to pay for a tethering plan if you are using your phone as a hotspot, and that is (for example) $100 plus $30 for mobile hotspot on Verizon. If I'm only using the iPad to tether, I'd much rather it be a plug and play direct connection, rather than something that is going to run down your phone's battery while you are trying to surf on the iPad.
In general, you should still be able to jailbreak and tether for free, especially if you are on a tiered plan.
Providers have gotten more aggressive about this, but everyone I know (myself included) have managed to "slip under the radar" to this point.
Also, as someone above pointed out- I can't remember a scenario where I didn't have both iPhone and iPad so 3G seems like overkill. I very, very rarely even tether my iPad (only at airports w/o free Wifi) since I can't think of things my iPhone can't do in a pinch (when I'm outside wifi range)