My daughter and I are planning to go to DHS in the afternoon of our arrival for the Marathon weekend.
Whether it is worth it or a waste of a ticket depends a lot on what time you would be able to get to the park and what kind of ticket you have. As a PP mentioned, if you are planning at least 4 other park days on your trip, the marginal cost of one more day is relatively low.
If you are planning to get
Genie+, as we are, it should be possible to get in 3 or 4 good rides with LL that would otherwise have fairly long standby lines. If you stay until closing you might also be able to do a couple of attractions standby as the lines dwindle, which often happens at DHS late in the day.
If you do get Genie+ it would be a good idea to try to get one of the most popular attractions right at 7AM. As is often reported, the LL slots for Slinky disappear within a minute after they become available. But, if SDD is a priority, and you can't get a time that fits your schedule in that first minute, I wouldn't give up.
I occasionally "practice" with the Tip Board by signing in at 7 AM EST to see how return times disappear. Pretty much without fail, some time a minute or so before or after 7:10 AM, times for SDD become available again. Whether this is because Disney has released more slots, or because slots are returned from the pool from people who never completed (or cancelled) a slot they clicked on right at 7, these times often seem to stay available longer than the during the initial crush.
Some days there are spurts of availability again close to 7:20 and 7:30. By 7:30 the availability seems to be pretty much gone for the day except for an occasional and random slot that pops up and disappears almost immediately.
I wouldn't count on getting a convenient SDD slot after 7:30 so, if I couldn't get a suitable time by then, I would move on to a backup, probably MFSR, which is the second most popular Genie+ attraction in the park. It doesn't take long for the return times for that to get out into the mid afternoon.
With a first selection at 7 AM, and then second and third ones at 11 AM and 1 PM (assuming a 9 AM opening), you can be armed with 3 good LL reservations before arriving at the park, and might be able to add more later depending on the crowd that day.