For many years I have been involved in the maintenance of Aircraft batterys. Batteries designated for 'emegency use' on aircraft are maintained, whilst off the aircraft, on a constant 'trickle' charge, using a charger that is not so different to the basic function of an opitimate. This ensures any emergency battery issued to an aircraft is ready to do its job. This is what the optimate also does, ensure your battery is ready to start your bike. However, these aircraft batteries are also subject to 'capacity testing' at specific intervals to ensure they provide 80-100% rated output should an emergency happen in flight. This is carried out on special to type testers and not something an optimate could hope to replicate, and yes, batteries that have been on trickle charge and then come up for a 'capicity test' do fail to pass the test.
Non emergency batteries for aircraft are typically charged, cap testesd, recharged and then can sit around for 3 months before being put to use. They are just as prone to not being up to spec as the batteries that remain on trickle charge.
So IMHO, and has been pointed out already, if you use your bike infrequently or have an alarm they are good. If you use your bike every day (and ride further than a couple of miles) then there is no need. They will 'usually' ensure your bike always starts, but cannot detect a capacity caperbility defect and could leave you stranded after a long ride due to a defective battery, as opposed to your bike not starting in the garage.