For what its worth, there is no simple answer to this questions. There are program directors that have strong feelings on both sides of this issue. Our PD is a very big name in the field and they don't influence him or his department in the decision making process in any way except that it can help with regional bias. Other PDs seem to view them more favorably. Having gone through the process and watched a couple rounds of candidates do it my impressions are:
1) They generally do NOT seem help people get interviews into top programs that they wouldn't otherwise. There are always exceptions and some programs are better than others, like Yale. But in general, MGH, Penn, Memorial, not so much. They have tons of aways and limited interview spots. Midlevels are a different story though. They are very useful for getting interviews at mid level programs.
2) There is regional bias and they can be very helpful if you are determined to be in a particular region that you have no obvious affiliation with.
3) You can learn a lot on them but Im unconvinced that information really allows you to impress anyone that frequently on the interview trail. As a student you don't really know that much (even the studs) about rad onc. PDs and departments are looking for people with potential and they use CVs for that. Virtually all the interviews are for is getting to know you as a person and if you are a good fit for them. Away rotations can be a good learning opportunity, but that is more for your own enjoyment and personal development, don't realistically expect that little extra knowledge to give you and advantage getting in that often.
4) Some people say it shows dedication and interest in the field. Again, maybe maybe not. Our PD laughs at that statement and warns that is a dangerous game. In his mind it could also make competing programs think you are too interested in a different program and could hurt you (sounds a little out there to me, but he has said it on more than one occasion). Strong applicants do just fine without them. I did zero and got >12 interviews including 5 of the top 10 programs when I went through. I was far from alone in that category.
Putting it all together my two cents would be there is no right answer for everyone. I think they have great educational value and can be fun but I am not convinced that they really help people match as much as other users might think. I doubt they ever hurt anyone but unless you know there is a region or program you are dying to get into, the only reason to do them is if you really want to. The other stellar reason to do one is if you are unsure if the field is right for you. Finally, you can get a feel for the atmosphere of different programs which can be useful, but there are 80ish programs and at most you will see 3. If you know this is the field for you and have compelling social or financial reasons that make it impractical don't kill yourself to do them. Just my thoughts.