Yeah, my reaction was WTF were they doing? Sure, they may not have realized that it would be there last appearances but it was nice to get them back (particularly Benton) after a break and then have them do pretty much nothing! And no Brig?! It was all very strange. Almost better to have eliminated the UNIT connection completely.
The Brigadier would have had Colonel Faraday's role in Android Invasion if Nicholas Courtney hadn't been booked up for a stage role (though it was only 9 weeks since Terror of the Zygons onscreen, Zygons was filmed for the previous season so he'd had a longer gap since last working on the series than it looks, and couldn't pay the bills by keeping himself available just in case).
The Brigadier was to have been in the original six part version of The Hand of Fear that was then planned to end season 13 (which would have ended with his death, though it was deliberately set 10 years on from the 'normal' UNIT era so he could have still returned in stories set before that). When that fell through and was replaced by The Seeds of Doom it seems likely that Major Grierson and Sergeant Henderson were originally meant to be the Brig and Benton, though there's no proof of that that I know of (and as a sidenote, Henderson dies, so would Benton have died too?).