I guess that like the cost of cruising "it depends." Mostly it depends on where you cruise. As pointed out there are areas where AIS is of very limited use - primary because few other boats are equipped. In those areas where there is high usage a receive only unit can help you scope out what is going on - helpful when approaching ports like New York or Miami or transiting areas with a lot of ship traffic like the Delaware Bay. As I pointed out in my previous post my experience is that larger ships have difficulty figuring out exactly where you are from a Lat/Long. I doubt they calculate CPA and time to CPA. The advantage of a transmit AIS is that their chart plotter gives them all of that information. It also gives them the name of your vessel so they don't have to call the boat at "approximately Lat/Long whatever."
With a receive only AIS you know that other ships are out there, their name, course, speed, CPA and time to CPA. This helps the Mark II eyeball locate them. With a transmit AIS they know you are there and they have the same information about you. It makes communication easier.
If you have a reasonably reliable full time watch in the cockpit or you are operating where there are not many AIS equipped ships one might take a pass. Or choose to invest in something else.
Risking the ire of newhaul and MikeORilley et al let me point out two items:
1. You used the term prudent.
What is prudent
for some may be overkill/inadequate for others - depending on budget, cruising style, cruising location, appetite for risk, etc. I am fortunate that I had enough money before I retired to have a pretty complete electronics suite - not everyone needs one or desires to own one. Over the course of some 15,000+ NM of cruising I have been relieved at some time that I had each element of the system on board. Did I need radar in the Caribbean? No. Did I need it in Nova Scotia and Newfoundland? Yes. My AIS came in handy when crossing the shipping lanes on my approach to Portugal. Watching 25 ships alter their course to give me 1 NM CPA was a great relief. I was coming off 8 days of 24/7 single handing in the Atlantic Ocean and I was tired. It was nice to not have to call each ship and say "do you see me?"
2. If you look at modern marine electronics the core of any system is the chart plotter - now properly renamed a "multi-function" display. Most modern multi-function displays are not only the core unit but the most expensive single item in the budget. Once you have the multi-function display the peripherals are less expensive e.g.
Transmit/Receive AIS $500
Depth Sounder - a transducer alone maybe $150
and so on.
Usually integration will cost a bit more - my VHF radio integrates with my multi-function display. It lets me push a button to call a ship that is displayed as an AIS target. Is it necessary? No. Is it kind of cool? Yes. I think the added integration cost was about $50.
For those on a limited budget - and we all are, our budgets just vary - the order and need for various electronic devices varies a great deal. The single most important instrument on my boat when I crossed the Atlantic was an MF/HF radio (OK, after the GPS.) I could get weather, report my position on a daily basis, and chat - important when you are at sea for 15 or 20 days alone. Useful in the ICW? Not at all.
We could assist you in your purchasing plans a bit better if you told us a bit more about how you intend to use your boat.
Fair winds and following seas.