I'll be traveling to Argentina with my wife and with our little girl (3 months old).
When taking a taxi, is it necessary to use my toddlers seat (and then we'll have to take it with us) or can my wife just hold her ?
There is a question exactly like yours on Trip Advisor. The gist of the answers is: Noone cares.
Taxis don't have car seats to offer and you're not required to have them to travel with kids, so your wife holding your 3 month old will be just fine.
Normally most countries(and states) exempt taxis from child safety seat requirements.
It's not legally required, but obviously it's safer to use one if you can.
That said, we've found using baby carriers like Baby Bjorn to be a pretty good compromise. They're not as safe as a car seat, but much better than relying on your arms and much more convenient to carry around. They're also a lifesaver at airports when waiting around in immigration lines etc.