I personally think they have a few more uses. For example, with the .52 Gal Deco (and probably the Aerospray, I just wouldn't know myself), you can send it out ahead of you quickly before shooting at someone and put them between a rock and a hard place. A good number of people seem to choose the rock and start shooting at you (which does make sense in the heat of the moment, since a squid can do a whole lot more than a Seeker), but can't dodge both the Seeker and your fire. Others might stay in their own ink and avoid the seeker, but if the area's not covered in theirs, it should be a little easier to find them.
They can also be used to just to hold people off for a bit by throwing them down a path, or to surprise people by sending them up a slope. (Arowana Mall Splatzones is a great place to do this, they can really jump up at people when they reach a zone) And they can be pretty useful for taking out people teaming up, if one of them doesn't avoid it, the other's dodging doesn't count for anything.
I feel like Seekers are hard to use well because it's so uncommon. There's one Slosher, one Roller, two Shooters, and one Blaster. (technically three Shooters, but Blasters are pretty different) If you prefer using a certain main weapon, you don't have a lot of options, and there's quite a few other kits that are much more appealing. I use the Carbon Roller Deco myself because I liked Seekers in Hero Mode, but it likely would have been better for me to just use the classic. Burst Bombs have the transportation use (plus inking walls!) and their own uses for attacks, and the Inkzooka is a special that you can actually use. I don't know how different a Tri-Slosher is from others, but it either has one alternative like the Carbon Roller or five, and there's plenty more Shooters and Blaster to choose from.
But I totally agree, since transportation with seekers is almost their main purpose, they would really suck on a Brush. The only benefit swimming after a seeker has compared to a brush is that you can recover your ink, which you usually need to do after using one. Sneaking can sometimes happen, but not really. Brushes use less ink and have more control over where they