I give my 2 raw-fed Rotties kefir also, but it's not for any particular problem, just because it's a natural probiotic and I figure it's good for them. Can't say I've noticed that it does anything in particular for them.
I've done Purina Fortiflora when we've been dealing with digestive troubles, though it's too expensive to be an all the time thing. I've used it both for a bad constipation episode with our eldest dog and a stomach bug both boys got a year or two back. Since they're transitory issues I can't really say how much of a difference it made, if any, but it made me feel better at least. There's enough scientific evidence for probiotics being beneficial that I'm happy to throw it at a problem like that just in case it does help.
The raw grinds we feed our younger dog (our older dog tends to get too hard stool on them so he's all kibble) also contain varying amounts of green tripe, which is also supposed to be a good probiotic source. He does well on the food but I can't say for sure if the probiotic element has a specific noticeable impact.
Yes, two of my three take probiotics daily. One has recently been diagnosed with a bit of IBD & was moved to a low fat diet with daily probiotics added, just for good measure. So far, so good (going on three months in to this routine now)
The other one tends to be a bit of an indiscriminate eater when she's out roaming around in the woods on our property (she has a way looser definition of 'food' than I do! lol) and now that she's getting older (just turned 10) also has some increasing issues with 'empty tummy, acid refluxy urping) so she gets a daily morning dose of Pepcid, a snack at lunchtime & probiotics once a day. Also, so far - so good.
If digestive issues are temporary or ongoing, I feel that using probiotics falls into the 'might help, can't hurt' category. Also, if antibiotics have been given for any reason whatsoever, at least a couple weeks to a month of probiotics are pretty important (in my opinion & experience) to get everything back on track digestively.