Kits are also for people who don't feel the need to do the same homework twice. It's all about money versus time. Do you want to spend the multiple weekends making your own? That's great-lots of folks enjoy the challenge! But also maybe you just want to install the kit this weekend, then go camping in the rig next weekend. Kits will help you do that, and they might even come with all the parts in one box so you don't have to scrounge for 'em. Nothing wrong with that approach, either. Half the time, you end up saving enough time or more, to equal the time you spend working for the price of the kit, versus saving money and spending a lot of time duplicating what someone else has already done. With a bunch of trips to the parts house, forum posts, online parts purchases, etc...
I've done it both ways. A well designed kit is worth its weight in gold.
