Bob - as indicated, the common opinion seems to be that the garden-hose style seal is required to ensure that the oil is indeed properly filtered. Nevertheless, there have been many reports of owners like you, discovering that the seal was missing for longer periods of time. This may have been caused by the old one falling off (although they are usually on pretty tight) when changing the oil and filter, without the mechanic noticing this. The fact that filter kits usually don't come with this seal doesn't help.
Now for the question of exactly what influence this seal does or doesn't have, I would like to hear some arguments. Does anyone know whether the oil comes into the filter from the outside or the inside, and whether it comes in from the top or from the bottom? Is there so much pressure inside the canister that, if the seal is missing, (a lot of) the oil is likely to pass around the filter, directly through the space between the top of the filter and the bottom of the canister holder (the space that the seal fills)?
If the usual engine diagnostics (compression, leakdown, oil pressure) don't reveal any real problems, I would not wrory about it. Just fit the seal (I buy 5 at a time and replace them maybe every 4 oil changes) and note whether there seems to be an influence on the oil pressure.
Cees Klumper in Amsterdam
'69 white 280 SL automatic