Paul, it's fine having your own radar and looking out for yourself and that's generally what I do but there are times when you want a vessel to know you are there and to allow them to see your likely intentions.
I guess, to some extent, it depends were you are boating and whether the commercial traffic is a risk to you and you to it.
Here, in the Firth of Forth, the access to the firth is from any direction and into two chanels which converge into one. Vessels travel roughly centrally and then split into two channels and then one of these splits again and the estuary is generally narrowing. There are various anchorages so vessels also travel outwith the channels. It's seriously spooky the number of times we would have been in a collision with commercial traffic if we had done our journey without paying attention.
Now, it's fine me knowing the other vessel is there but if I was in the commercial vessel I'm sure I'd like to know if there is traffic that could be a risk to me. Giving the skippers of other vessels peace of mind by being aware you are there and giving an indication to them of your intention by your actions, I feel, is good practice.
Should you become disabled in a shipping lane, you'll want a vessel bearing down on you to be aware you're there and you'll want them to know that in plenty of time. Given the speed commercial vessels travel, seeing you just a mile off their bow is not going to be adequate.
Given the poor performance of passive reflectors, I feel an active transponder is a worthwhile thing to have.
Whether the commecial folk are looking out on X-band you have no control over but they should be, especially in restriced waters where they will know X-band radar is more likely to be in use.
Contacting a vessel which may be bearing down on you is another issue. The crude way is a flare and hope someone is looking out of the window but to give a large vessel sufficient warning to take evasive action means you are going to have to release the flare fairly early. Shouting on the VHF and hoping they are monitoring ch16 is another way. Calling the local traffic service and giving them your position they will be able to alert the vessel for you and give you the vessel's name so you can speak directly. Again, time is important.
AIS may be a solution because you will have the vessel's direction, speed and MMSI. This is the route I've taken and, fortunately, I've not needed to use it in anger. Hopefully, I never will.
However, I'm hoping AIS will prove to be generally useful in another way because on a number of occasions I have misread the direction of travel of a vessel. This has happened when the light is such that the vessel is a silhouette and I'm viewing it at an angle. It has been difficult to tell whether it's moving, particularly if it's moving slowly, or whether I'm viewing the stern or the bow. Stopping and viewing the radar to plot its track is one way but when one's helming, decisions are made by what one sees and unless something alerts one to a problem then the incoming information is processed without thought. If some of that information is faulty one may be in for a surprise!
It's a surprise I'd rather not have.
Alerting other vessels brings me on to the crappy design of VHF radios....another time.