"Outside of the DD2 Invasion event, you now have a limit of one sentry summon at a time"
Well, that's a shame. Is there any reason in particular that this change was made? It pretty much scrapped my main summoner (had Queen Spider Staff, Staff of the Frost Hydra, Lunar Portal Staff, Rainbow Crystal Staff, and the Clinger Staff). I would have these summoned around the area I was in, in different locations.
I'll start by saying that Clinger Staff, for all of its numerous weaknesses, isn't a sentry summon, so its not influenced by this change.
But moving onto the meat of the issue-
Sentry turrets were, until this update, a very limited pool of items. There were only 4 of them, and they slowly snuck into the game across a period of almost two years. There were never enough of them to make a huge impact on the game, and by the time you had access to the Lunar Portal Staff and RCS, the Queen Spider and, to an extent, the Frost Hydra, were somewhat outdated.
Because of the low impact of these items, and the low number of them, the fact that there WAS no limit on them sort of slipped past the radar. It wasn't "intentional" that you could use all turrets at once, so much as it wasn't even considered. The moment the higher ups realized the impact this unintended element could have, especially with the incoming addition of a dozen more sentry turrets, it was removed. This doesn't mean that it was "the addition of more staves that led to the change". It was merely that, while adding these staves, this design was given more attention and it was decided that it wasn't what was intended. One way you could state it is, "If we had known at the time of putting the Frost Hydra Staff into the game that there would eventually be a bunch of them, this limit would have been there from the start."
I've mentioned before that we've included armors and accessories which actually buff this sub-class of the summoner (though its not compatible with a full minion build) so that you can actually have MORE turrets than you could before this update, with significantly higher damage. And that's a gear choice you can choose to make or not to make, as with any choice in the game.
Long story short, it wasn't intended that any player, of any gear selection, could use all 4 Sentry Summons at a time without any sacrifice in terms of equipment. It got this way because of the low number of Sentries and the long period of time over which they were added, and now that more attention has been paid to it, it has been changed, with some buffs to potential as well as the removal of the free multiple sentries.