...because you have no idea what kind of healer and DPS you'll end up with? A geared healer will be just fine keeping you up anyway, but if you get a fresh 50 in tionese+recruit he won't, and even if he does it's because he's working his *** off.
I do sometimes do that myself, but while having a full tanking gear and field respec and start out by asking the healer if he's fine with it or if he'd rather have me respec.
As I said, I rarely PUG but when I do if I've got a newbie healer I just hold back and let the tank do his job. Its a lot less entertaining but it works. That said, there are no (compulsory) fights in the FPs aside from LI that require much healing on the tank. Most of the worrying stuff is interruptable (Ironfist), randomly targeted (Darth Severin) or someone standing in stupid (Vokk). Frenzied Behemoth in Kaon is the only one I can think of but that's a taunt fest anyway due to the semi-constant threat drops.
When 2.0 hits and we have the gear reset tanks and healers will become more important and with the changes to Shielding I suspect tanks won't devalue as quickly post 2.0.
Anyway this discussion has inspired me to grab my normal healer, both spec DPS but queue our normal roles and see how we go with unsuspecting DPS. I'm pretty confident we'll be fine and most DPS won't even notice unless they check my Cell or have enough situational awareness to notice us doing things we shouldn't be able to in our proper spec like IA procs and FiB.