Alc.pigs, all I can say is you've either been lucky, or never noticed the digestive impact of a full dose of Baytril.
I have, what comes out the back goes from normal to varying in size, and more noticeably consistency. It's not at all unusual to see mucous between poo, bubblews of mucous, and other anomalies. All of which appear soon after Baytril but weren't repesent before. The logical (sorry to espouse logic here) conclusion is the antibiotics have a negative impact on the normal balance of gut bacteria, which increases intestinal mucous production as well as reduced digestive capability. This is because antibiotics, especially broad spectrum ones, *do* inhibit or kill (depending on the antibiotic in question) gut bacteria required for healthy digestion. The very nature of an antibiotic *is* to kill bacteria, and the wider the scope of that killing the higher the chance it will also kill bacteria you don't want killed. A narrow spectrum antibiotic will have less impact, maybe none, but is also likely to be less effective against the systemic bacteria you are trying to use it against.
The reason we don't give oral penicillin (the fear from oral administration may be preventing using it in injectable form, but is understandable) is it is *so* harsh on the digestive tract it can and frequently will be lethal within a very short space of time. It's not a huge leap to conclude that other antibiotics can have similar, if not so severe, effects.
The use of probiotics in humans on certain antibiotics is not unheard of for similar reasons. Although our digestive systems aren't as delicate as that of 1kg herbivores, antibiotics still aren't the kindest thing to put through them.
As probiotics invariably contain levels of electrolytes and certain vitamins, I cannot see any reason to refuse to give them if only for the secondary benefits of these additions. A 100g tub of Avipro for example will last for far longer than one regular treatment of antibiotics for one piggie, costs less than £10, and helps boost vit A C and E levels in the diet as well as containing electrolytes, a level of fibre and of course certain bacteria.
Even if I didn't have logic and experience to back my opinion up, I would rather go with "a good idea" that certainly won't hurt, than decide it's all bunkum and possibly be denying my critters something that may be of invaluable help.