Yes. On bents they are. Even more so than rear racks.
In fact, I’d rather have only underseat racks if I could only figure how to put two of those dippy bike baskets — the best/worst of which I have on my rear rack — on the sides. Maybe someday soon I will (w/ apologies to my fashion-conscious friends). But since I like having a bike basket w/ bungy netting for fast and easy storage, I’m looking at spending more time w/ a rear rack. (Tempted to get a still bigger basket on the back, in fact. Shelly Mossey and his NYC bent-based courier service better look out.)
My knock-around city bent is an old Rans Rocket. Great urban bike, but serious balance problems if I load up the rear rack, most of which is actually outside the wheelbase (beyond the rear wheel), causing the front wheeeel to elevate if you have something heavy back there, like a heavy lock and a couple laptops. Normal stuff, in other words. But makes for some scary sh*t when you hit construction zones at speed, w/ that front wheel leaving the ground every time it hits a fresh bit of asphalt laid New Yawk-style.
Problem solved: put the weight in side panniers on my bent — I mean scratched and bended, not recumbent — underseat racks. There are times I’ve considered stripping off the rear rack, but I like the rear light bracket (making it all one heck of a large and heavy light bracket!) and I can put big, puffy and light things back there in the basket like…boxes of salad greens, swim gear and warm jackets.
All that to say, I’m all for saving money. But not by trying to go w/o the underseat rack. (Less aerodynamic than a rear rack? Well, I suppose so. Who cares, I’m in traffic.)