You are talking about extra steel and moving parts pinned on in place of the bucket and the bucket or whatever mounts beyond that, it does make lifting capacity less due to the weight of those items, as well as the fact it moves the bucket an inch or two farther out from the pins creating additional leverage that works against your lift capacity... You can lose up to about 250# off the rated spec going from pins to SSQA.
Personally, I'd keep the pin on bucket around for large heavy jobs and get the SSQA with attachments for everyday stuff. The breakout force is like 2600# and the lift is like 1400#. For everyday stuff you can lose a couple hundred pounds and it's still a lot better than a wheelbarrow; and when ya need to work with heavy stuff ya drive the pins out of the SSQA adapter and into the original bucket. If using lighter weight items on the SSQA like a lightweight set of pallet forks, your capacity will go back up due to the fact the implement is lighter.
It is really about finding the balance of what will work best for you.