Very common assumption from an iOS user. Your on the right path but let me explain.
MotoBlur, HTC Sense and Touchwiz goes beyond what you see from Facebook Home. The intent of Touchwiz for example is to tightly integrate everything from widgets, software, switches, fonts, device drivers, battery tweaks and system wide optimizations. The stock Touchwiz Rom from my Note 2 approaches 1GB in size. A Vanilla Jellybean ROM is under 200mb. If I use a Cyanogen based ROM (the intent is to have stock android) I lose the cool features of my S-Pen, battery life (might not be good as Touchwiz) and none of the services like S-Note or Air-view will work. You may even need workarounds for GPS and Bluetooth because those drivers are so tightly integrated in Touchwiz based Roms.
If you disable Facebook home all that remains is stock Android. HTC Sense is not running under it which for some is actually a good thing. If this phone was running HTC Sense you would see a performance hit. Hence the reason why people mod Android phones for a stock look and feel with better performance than say Motoblur.
And finally I can actually download Facebook home on my Note 2. No different from downloading a Windows 8 launcher or a iOS launcher from the play store. I can completely change the look and feel within seconds without the need to "jailbreak" my device.
And don't pay attention to my battery life. I'm going on 26 hours and I haven't swapped it yet.