Well, you need to unless you want an unreliable mod that could brick your box if it ever goes unplugged, and you'd need to buy extra stuff to do the mod anyways.(Which would come up to around the same price of a modchip IIRC)
That's for a really really
outdated mod. The clock thing isn't an issue anymore.
Not to mention the lack of support for replacement HDDs(Which I forgot to mention earlier), a new BIOS will let you plop any old drive in instead of worrying about HDD locking and such.
Yep, the locking thing is true, but I'd hardly call it lack of support. Locking a drive to an Xbox isn't really hard, and can be undone. It just takes slightly longer for the initial install, but after that it's exactly the same. It actually used to have the added benefit of being able to use the drive in stock mode, and even go on Live with it assuming the console hadn't been on Live prior.
This is why it really depends on what you want to do.