How useful are the +10% CEs from the shop?

My answer to that is quite simple. Chaldea Lunch time. Increases bond points 10% when MLB and saves you lots of time from reaching bond 10 which takes 5-6 (?) months of playtime that you spend with that certain servant around.
What does this mean? It's to be efficient and save time. I mean how much time do you even have to reach the London mystic codes 72 million plus EXP? Makes each treasure vault run give you a million of QP when you desperately want it and such, but honestly just save it for Chaldea lunchtime. Nothing more nothing less.

Chaldea Lunchtime is prob the best Bond CE so far as it gives a +10% as opposed to the other CEs which only give +50.
As for why is it better to get instead of Exp/Fou, it's because the CE is only available for a limited time, while the Exp/Fou resets every month, and also they can be farmed quite easily as you play more. The Exp/Fou can also be obtained by trading in event mats, and occasionally, lottery events.
With 1/2 AP events sometimes you'll farm the hell out of the hands and net alot of EXP cards, so while doing that the bond CE in the back will give you the added bond points
You can get Exp/Fou every month, but when you miss the CE, you miss it, and the only way to get it again is to trade rare mana prisms for it at the cost of 4/5 star servants.
Since you're quite new, it may not be that bad to spend on Exp/Fou but if you're planning to play this game for a long time, you might regret not picking up the CE, once your servants are max lvl/fou'ed.
Also free FP because during a lull period everybody will want Bond/QP CE supports while they farm

I think the CEs are pretty good when you want to save time grinding for those Mystic Codes exp and Bond points.
one of my regrets in playing this game aside from not playing it earlier is that I missed the Mona lisa CE back then(which is also tied to me not playing F:GO earlier).
That said, I personally don't buy them coz; a) I always spend my MP on Fou-kuns, tickets and ember cards(yes. I'm one of those people) and b) I'm not in a rush to get Max bond points or Level capped MCs or Master level. I kinda just want to let it happen. And the fact that I'm always just farming for embers and skill gems when there's no event. Maybe it'll change once I make a crap ton of progress with servant level and skill levels.