If you plan it as dive air as opposed to burning your lifeline I don't see where it's any different than a stage bottle.
Being a single bottle it's only good for the first dive but maybe the OP does a lot of single tank outings and that's just fine. If nothing else it gives him what he needs to make a meaningful first, perhaps deeper dive.
I agree that longterm for local use getting a larger primary cylinder is the cleanest and cheapest way to go. The pony has the advantage of being transportable to a dive destination that offers nothing larger than an 80.
cjprzed, Be sure to keep her in mind in your consumption plan remembering that only your primary cylinder allows you to support her if needed. I'd do the pony time before your primary cylinder gets below 50% that way you always have ample air accessible to her.
cjprzed, Have you also considered sharing some of her air? My wife will use some of mine at times and with planning it maximizes the dive while building coordination. This worked out great on Bonaire. We'd head up current at a chosen depth, as an example 60 feet. When she hit her turn pressure (50%) she'd take my alternate and we'd perhaps go another 10 minutes. When I hit mine we'd turn back and return at a sallower depth, say 30 feet. Heading down current and being shallower we'd both get back with ample air and always had plenty do deal with an issue if something came up. The pony's a fine solution but this is another option for a second dive or if the pony is not available.
The reason it is frowned upon is that some people are not open minded enough to see a pony bottle as anything but a sacrosanct emergency air supply.
Pete