Tech trading was a bit too profligate in the early days of the game, but with the new mechanic of not being able to initiate a new diplomatic contact until after a "cooldown" period, it feels better balanced (IMO). I'm not sure, but I think the devs might have tinkered with the code over time, to make the races less likely to trade the more strategically valuable techs like military tech and extreme planet colonization.

I usually play with tech trading turned on, because it's a way to rush-buy a colony ship or sensor scout by generating a little cash during the colony rush. I try to sell to minors first, to slow the spread. Later in the game, it's one more tool for diplomacy or manipulation. Sometimes I'll sell or gift a few techs to a race I'm trying to use as a buffer or distraction against a more dangerous race, to buff them up. That can be fun, if you're careful to protect your more valuable research lines.

So I'd say leave it on, unless you're spotting something about it that really bothers you. I might get annoyed at tech trading if I consistently fell behind the other races in tech research, but I usually manage to keep up.

To say nothing of getting some quick cash from the other races (especially minor races) for the less useful techs. This can be important if you are still running a deficit in your economy.

Well, you're right about trading them one by one, but in DA, you have to make the most of your diplomatic relations, because in DA, you can't talk to them at any old time. Only with the Super Diplomat ability can you talk to them every few weeks, otherwise it's longer to wait.