My reasoning for Jailbreaking.... I wouldn't have even bought an iphone without it at all! IN fact, the iPhone 4 was my first iphone... and I waited to try one, because I needed certain requirements to be met before I shelled out 800.00 CDN for one!
My #1 requirement was a 'Today Screen' where I could see all notifications, ToDo's, Appointments, weather, missed calls etc all on one screen at a glance. The default iPhone method to check all of the above would require first unlocking the device, then scrolling through all my app icons to find the badges, then open each app individually, one at a time, to see the new information. <- THAT is a pain in the rump! Without LockInfo, I wouldn't own an iPhone at all! I'd have likely stayed where I was on Blackberry or gone to Android by now.
My second requirement was theming, because let's face it, the default iOS look is boring and haphazard! I'm talking about the home screen, and not the in-app look which I like. Things like a non-dynamic weather icon that always says 23 degrees and Sunny is a pet peeve, and while the calendar icon is dynamic, why can't the clock one be too?? Hey wait... it CAN be! Because I jailbroke! ANd my weather icon can be dynamic too... always telling me the ACTUAL current weather without needing to go into the app (unless I want the full 5 day forecast)
And the above tweaks... in addition to SBSettings, and a few other things I am running, do not affect my iPhone 4's speed or performance at all! Battery power, maybe just slightly because of the dynamic icons and lock screen info, but it's a worthwhile trade off IMO and I still get a full day of moderate use. For times when I know I need more juice, I slip it into the QYG Power external bettery power pack I have.
LONG LIVE JAILBREAKING

Oh and btw, I am not a super geek or even someone who knows a lot about hacking. I a just a girl who knows what I want my device to look like and do! And I can READ so I don't mess anything up!