It's still nice to see how many you can save up.
There is not very much incentive in a capped vault to get Mr handy's, they best serve a starting vault with caps, there are a few benifits to having them arround.
They can watch floors you dont have dwellers on and engage incidents before they spread, or help with invasions.
Their constant resource collection can help level dwellers faster.
They're neat and they float and.... robots, man! they're basically like a nice rug in your vault, pretty to look at and ocasionally fight for you.
If you arent wanting them just to have them in a maxed vault I dont reccomend spending the money.
Since you can get them in lunchboxes now there's no point in buying them at all is there?
I bought one just to support authors and assigned him on the floor with medbay so he can collect stims even when i am in the middle of battle with Deathclaws, which was useful when they started to appear (61 dwellers in my vault).
BTW: He has some really hillarious comments when wandering wasteland, but he is much more usefull inside your vault.