Yes, but not as much. None for light and medium buildings, 25% for heavy and 50% for hardened. Also, building hexes act as light woods hexes for purposes of determining LOS and to hit modifiers.
But damage is still applied as non-infantry firing weapons... unless you happen to be infantry. So, you're still only killing one guy at a time.
Unless you're shooting at infantry in an all-glass building you're probably not going to see them. If you can't see them, how are you going to hit them? There's blind luck (pun intended), but that's about it. BattleTech sensors are slick, but we're not talking Star Trek uberSensor systems.
Hello: Open faces and mouths equal heat source. BattleMechs have IR sensors, and their computers are pretty slick and can probably pick up on that. In fact, I recall reading such things in various novels, like Heir to the Dragon and the first of the Clan invasion books.
Or, are you referring to point-blank attacks from hidden units? As soon as someone powers up a weapon, pokes out the barrel or exposes part of him or herself, any armored unit with a decent sensor suite is going to notice. (Hence why attackers that use the point blank attacks are revealed as soon as they do.) Are you suggesting that firing back isn't going to catch more than one someone with his head still up?
A 30 meter hex isn't that big.
100 feet is still 100 feet. Most houses are hardly that wide. Most don't even come close. That is, unless design practices have changed in 700 to 1000 years. (Which I can imagine might be the case.)
Sub-urban areas would be chalk full of tiny little houses, again assuming that living spaces are still cheaply designed. See, I too live in a small town, and I'd be hard pressed to find more than a handful of buildings that actually take up a full hex. I happen to be sitting in one right now while I type: The Library.
In a city, that's something else entirely. Same with industrial areas. My town's Wal-Mart is a multi-hex ordeal. Don't know if it's exactly tall enough, though.
Still, this has made me momentarily reconsider whether my interpretations of 'buildings' needs reconsideration, along with infantry.
But, it does nothing for the fact that one of three dedicated weapon systems to a particular role - anti-infantry - don't really fulfill it all that well where it's needed most, and yet they get accolades for it in the fiction. (I hope people realize that they function the same on tanks, too, and this is not just about Mechs, but armored vehicles using these weapons in general. That's right. Your precious tanks, too, are unable to dish out the punishment that they should.)