Having one base is cool and useful for a variety of reasons.
Recruiting all the other settlements, well it's xp and a boost to the "like" meter on some companions, and hell, you know you were going to kill those raiders or whatever anyway. Actually building them up though? What's the point?
Not that it's that difficult anyway, even without LL1. Just roll in with enough mats to cover (10+Base CH) settlers for food--use carrots especially for the shitty ones with no room to plant--water, and beds. If there's a convenient central area, slap down a few turrets on a high point to help with invasions; the heavy ones are the best bang for the buck, so to speak. Build that scavenging bookshelf thing that increases the random junk they produce and assign someone to that as well, so at least they might sort of pay you back for the effort. 1 settler on that, 3 max on food, and you're all set. Not really sure what the rest do. I guess you can build guard posts and increase defense to deter invasions that way if you really want to.
The biggest thing I'll miss when I play a character that doesn't have 6 CH is having it be so easy to do all my dialogue at 10 CH. I'm pretty sure it's impossible to fail a persuasion check as well as being a decent boost for bartering.
Is there a reason to not equip every settler with gear? I noticed when I gave more settlers armor + weapons/ammo and put down a new defense wall, one automatically went to it without me having to assign it which was kind of nice.
No, if you don't mind the hassle of collecting equipment and gearing them all up, go for it. For extra fun, give them each 1 nuka grenade. Any NPC with even a single explosive in their inventory has an infinite supply in combat. Get revenge for all those times some raider threw molotovs at you over and over. They have infinite ammo too, as long as you give them one bullet of the correct type, so you can have fun with all the miniguns and shit you've been hoarding for a rainy day. Random settler with a fat man--what could possibly go wrong?