We've gone from thinking of software as a "durable good" to evolving under selection pressure. This certainly applies to malware and blacklist-driven scanner countermeasures, which are assumed to become either extinct or irrelevant over time.
Who needs old scanners?
You may want to keep an old scanner if it still detects stuff other scanners can miss, or if it was the last version that ran in your environment - though as an "extra" on-demand scanner, not as sole resident protection, of course.
For example, Kaspersky's CLI scanner no longer runs under Bart, and I haven't adapted AdAware 2007 (which I don't particularly like) to run in Bart either. There are still manual updates for AdAware SE, so that's still "current", and Kaspersky CLI still works in Safe Cmd. However, I may want the safety of formal scanning with Kaspersky CLI, and for that, I'd need the last "old" version and updates that still worked from Bart.
Another example is McAfee's Stinger. Just as an "old" Kaspersky CLI may find stuff other updated scanners will miss, so it is with the even older Stinger - it's particularly good at catching TFTP-dropped malware and some bots, both of which are likely to be found in an NT that has not patched RPC against Lovesan et al.
F-Prot's DOS and Win32 CLI scanners are also discontinued, i.e. no further updates, but are still useful. Specifically, these scanners will often detect "possibly new version of ... Maximus", and sometimes that rather loose and false-positive-prone detection still finds things others miss. These scanners also find other false positives unrelated to Maximus, so handle with care.
Does old malware matter?
Firstly, you may encounter vintage malware on vintage systems and diskettes (e.g. boot sector infectors on old DOS or Win95-era PCs, old MS Office macro infectors in "documents" from old systems). Malware of that era were mostly self-contained and fully automated, and often had destructive payloads, so they will still bite... so you'd want to detect them.
Secondly, think of the spammer equivalent of the guy who still uses a PC built from old parts running MS Office 2000 on Windows 98, because these old feeware programs pre-date automated defence against piracy - i.e. no software budget.
A less-obvious feature of botnets is that those who own them, don't want folks controlling them for free. So if you want to send spam through a modern botnet, you will probably have to find someone and pay them.
On the other hand, old bots that are still in the wild, may have been cracked so they can be operated for free - or may simply pre-date the rise of malicious info-business and thus lack modern mechanisms to block control. In which case, our impoverished spammer may use these instead - so it may still be prudent to detect and kill them off, especially in the context of poorly-patched or defended systems (e.g. unpatched Windows 2000, no firewall, outdated or missing av).