Question about train to Salisbury from Waterloo Station

No drawbacks apart from the cost. I wouldn't expect an evening train on Saturday from Salisbury to London to be so crowded that you'd have difficulty getting seats.

The other thing to bear in mind is that while most evening trains from Salisbury to London are through trains from Exeter, most have extra carriages added at Salisbury. You can normally board that part of the train before the Exeter portion arrives, as it forms the front portion of the combined train.
Then when it arrives, it attaches to the rear.

So, unless something exceptional is happening in Salisbury, there are no obvious disbenefits.

Thank you for the replies.

I think the pressure to prebook so many parts of this trip (Westminster Abbey, some other sites, Eurostar tickets, etc) have made me have a scarcity mindset in rushing to prebook all aspects of this trip. Thank you for the reminder that not everything needs to be so strictly managed and that it's unlikely we wouldn't find 2 seats!

I did this day trip in Oct. 2024. You might find the info. from my trip report helpful. Take note of the no seat reservations on the train.

Today was our big Stonehenge day. This is something my sister and
brother-in-law really wanted to see. I had read about the Stonehenge
Tour on the forum and it really worked well for us.

https://www.thestonehengetour.info/

It involved walking to the Waterloo train station to take a 1.5-hour
train to Salisbury. I had booked us open return train tickets in
advance. There are no seat reservations on this particular train. Then
right outside the Salisbury train station, the bus picked us up and
took us to Stonehenge. The bus ride is not time specific, so you can
take any bus you like. We took the first one at 10 am. The check-in
process was a little slow as it seemed most people had not purchased
tickets ahead of time. (I bought our bus and Stonehenge tickets ahead
of time from the tour website linked above.) The bus dropped us off
near the visitor center where we had to show our tour tickets to get a
Stonehenge ticket. We then took the free shuttle out to the site. We
had a fantastic day!! Blue skies!!!! It was not crowded at all. And
much to my delight, there were sheep all around. I had originally
planned for us to see the Salisbury Cathedral afterwards, but it was a
long day and so we decided to just head back to London.

https://community.ricksteves.com/travel-forum/trip-reports/10-days-in-london-our-first-time-a-trip-report

Carrie, Thank you!

That is very helpful. Sounds like it worked out perfectly for your day to Stonehenge. I'll plan on the open return ticket and we can also see if we will have time to see the cathedral also that day.

Edited to add: Carrie, I just took a look at the photos in the link you shared. That photo of Stonehenge on that blue sky day you had is amazing!

That photo of Stonehenge on that blue sky day you had is amazing!

Yes, we were so lucky! I hope you have the same good weather on your trip.