Virus definition stay over time and as such take space. Antivirus structure their signature database for management purpose and and permit us, technical people, to be able to manage, repair, change and backup them. Being able to protect the integrity of the virus database engine is part of the security itself and a necessary monitoring activity (imho). I think it could explain a bit why antivirus program are sometime so deeply rooted to the machine it 'protect'.
As IT sec, it could be your duty to manage risk upon factor such as compatibility and abandonware, etc. Excluding those signatures would reduce disk space and reduce antivirus engine corruption, fragmentation, etc, and the risk would be managed, etc.