I've done it. Vox Pathfinder 15R or Ampeg G-18. I did a medium sized outdoor venue with two of the G-18s, probably 500 seats at tables. The other player in one of my bands will mic up a Champ, oddly, like Shredtrash, also for rockabilly/old country gigs.
Julian Lage does it, Margaret Glaspy does it, Alan Sparhawk (Low) does it, and Afie Juravenen (Bahamas/Leslie Feist) has done it. So yes, it works, just depends on what you expect to get from it.
Personally, I'd love to do it for a gig, but I'd probably wouldn't use one past Cafe singer/songwriter gigs. I have a lovely little Valco Supro Spectator amp with an old 8" Jensen that would do that with are, but any kind of larger gig I probably wouldn't. I'd rather just grab one of my Princeton amps, unmic'd or mic'd, and be incredibly satisfied as they are tried, tested, and true amps for my purposes while still keeping the amp small and grab-n-go ready.
An amplifier has no idea how big or how many speakers it is connected to. It only sees impedance. Therefore you can play though 100 speakers assuming impedance is matched and wiring doesn't become a substantial percentage of the total speaker impedance. Remember that speakers are 3 dimensional. They have volume. IIRC 2 x 12s is still less than a single 15. The only way to get volume and bottom from a small speaker is to increase the excursion, the distance the cone and voice coil can safely move. Speaker design has improved some over the years but much like Hot Rod engines "There is no substitute for cubic inches". It's a pretty safe bet that the larger the speaker you play through (assuming decent response and efficiency) the more you and your audience will like it.
I have often used an un-miked 2x8" Kay 550 paired with a late fifties single p/up dano/silvertone for gigs with a tastefully restrained drummer and a singer/guitarist going through a PA. In extra small venues I have gotten by with a Valco made Oahu amp with single 8 and Gibson es330t. In a small enough room an 8" speaker can sound pretty big. On paper two 8" speakers equal one nearly 11" speaker.