Rooting an Xperia is actually very simple. The descriptions are extremely detailed just to be sure that nothing goes wrong.
Your firmware will remain stock unless you flash a different one, otherwise it's left untouched during the unlocking and rooting process!
For the Xperia P
you perform the following steps as described in detail here
.For any other Xperia model take a look at the newbie thread for that particular model.
1. Unlock you bootloader using the description provided by Sony here
This only needs to be done once. And yes, you can
relock your phone if you choose!
2. Download Fastboot here
and extract the files
3. Download a kernel with root and CWM (ClockWork Recovery). I currently use this one
Put it in the same folder where you have fastboot
4. Turn off your phone
5. Plug your USB cable into your PC
6. While holding the Volume Up
key, plug in your USB cable in your phone. You're now in Fastboot Mode (blue led is on)
7. Enter the following commands in a terminal on your PC: fastboot devices
(should show your device) fastboot flash boot <name of kernel>
(e.g. fastboot flash boot boot_P-7.2.img
) flashboot reboot
The phone should now reboot to a rooted system. In addition, you have the CWM menu (lets you flash firmwares, kernels, etc. in zip format, make backups of your current firmware incl. kernel, etc. Look at the descriptions) and you can use Titanium Backup to get rid of all the crap.
Take the time to read the post over at xda-developers
I linked above, it's definitely worth it.
Btw, there are apps that need root permissions (like Titanium Backup), and make sense that way, but see what others say in the respective fora about any app that wants root permissions!
I'll be happy to help if any further questions come up (if I can).
Hope this helps clear up things a little