The MBs are great machines. So are the MBPs. If you can't see the difference, then you don't need a Pro. Actually, even if you can, you might not need a Pro, but some of us do.
For example, I need that Expresscard slot for an external SATA controller so I can get some serious drive speed on externals when I edit video (frankly I just never bothered with FW800--I'm not saying it doesn't have its place, but eSATA is faster and with a case from FirmTek I can swap in new drives for the cost of a $20 tray). Being able to swap my eSATA case directly from my tower to my portable is a good thing.
Likewise, since right now I'm using my MPB for my main machine a 13" monitor isn't going to cut it, and hooking up an external isn't an option. In fact, the 15" was going to be cramped to, so I have a 17" with the same resolution as my 20" ACD on my tower. That works for me, and is easily worth the extra cost. Hugely beefier graphics card doesn't hurt, either, depending on what you're doing with it.
Other nice touches like the lighted keyboard, auto-adjusting screen backlight, and very nice speakers are a bonus--the little touches that make it Pro.
Oh, and incidentally, if you've never put them side by side (I have), while the MB has a VERY bright screen, and is quite nice for a budget laptop, it's of far lower quality than the 17" MBP screen--the color rendition is much worse, and it's much more sensitive to viewing angle.