After a some thinking, the manned ack version of this weapon shouldn't be able to be used as AAA and AT, AAA only. This would be the best option.
However, if they were to make a "gv" version of it, towed 88mm, then the option to up it as an AAA or AT version would be do-able. When you select rounds like you do on a tank, it would have the AAA ammo then the AP/HE ammo. This would be more accurate, though I am uncertain if they had HE rounds when deployed in the AT role. I wouldn't think they would not have the option to use them but those of you who have access to books on the weapon, you would be able to say one way or the other. The downside to this would be to get someone to up a SdK to tow you to a spot.
The AAA version would probably see more use than the AT version since it's a lot quicker to up a tank vs having someone up a SdK to tow you when it comes to combating gv's. However, if you expect a GV attack, having the 88's deployed in the AT role could be useful. Another downside to the towed 88 is that it wouldn't take much to knock it out. 1 or 2 HE rounds from a tank, or a strafe from a plane. There is no protection for the crew after all. A well placed 88 will do a lot of damage however, and MAY be harder to spot vs a tank.
When it comes to the AAA version, well, yeah. You are likely to get strafed just like any other ack gun. BUT, you have the ability to down planes 3k and up with the puffy of death.

"Up a fighter. It's quicker...". Maybe, maybe not. Maybe I don't wish to up a fighter at the moment. However, if there are deployed 88's helping guard the base, then the fighters that do up can pay more attention to other enemy fighters and hunt for goons, or at the very least, the 88's help the fighters knock out the bombers (not many pilots will be able to down all the bombers before they reach their target after all). Of course, if the 88 crews killshoot themselves, then it's probably their fault. Firing at bombers when friendlies are making their passes....yeah. If you take the risk, you may end up paying for it.

It's up in the air with the towed 88, but the point is there is nothing wrong with having the option to use it. Something for later on down the line, that much is obvious.

As for the Manned Ack version, my previous posts about it (minus the AT part of course), could be do-able.