Pros

• I am still actively deadheading many plants in the hope of a more flowers from a long, warm, sunny Autumn – some hope editor
• Plants look tidier if they are lightly trimmed when deadheading
• Energy is put into the remaining plant rather than seed production. So plants may be better able to withstand winter and some will have a better established root system.
• Softwood that has no time to ripen will probably suffer in the first frosts so it is pruned out
• Dying flowerheads may rot or damage other flowers or leaves.
• Deadheading stops unwanted seedlings from prolific seeders

Cons

• If you want to save seed you want seedheads to ripen on the plant. Some will dry in a greenhouse or garden shed before being stored in an airtight container. I put seeds in small ex-mail order envelopes first.
• Some seed heads such as Honesty, Rose rugarosa,  Echinops and Teasels are left through winter for shape and to look attractive in a frost.
• Do not deadhead ornamental plants grown for their seedheads like Iris Foettisima or Physalis
• If you want to save seed or berries for birds and wild life do not deadhead
• If you want self-sown seedlings for a natural garden then select what flowers to leave to run to seed.