If you compare the solo with the double or triple attack to a group of 5 monsters instead, the defender (most of whom mark a single target reliably, two with effort, and only get to trigger their mark backlash once a round) likely is not managing to catch all of the attacks, so it's pretty comparable to allowing 1/2 of a double attack or 1/3 of a triple attack.
It depends somewhat on level; but most defenders can multimark even fairly early on. Sure, they can only trigger the "extra benefit" once (and even that's not always the case), but they can multimark. A fighter with two attacks or a close burst, a warden in any case, a paladin with a divine sanction (or even a divine sanction blast), or a paragon swordmage can all manage multiple marks. The paladin even get's the bonus damage once per creature. In any case, solo's are more vulnerable to all sorts of single-target tricks; it's not surprising this may be the case to some extent with marks too. Many effects simply target only a limited area or limited number of creatures, and that makes it easier to hit a solo with such effects than to hit a group of creatures with those things.
Also an odd choice 'nerf'. If the monsters are intended to work that way with the mark, then it's not a nerf at all. It's working as intended. It is hard to judge what the people making monsters are thinking, of course, but I can pretty much guarantee that at least some of them are fully believing it.
The rules on multiple attacks are quite simple; they are seperate attacks. Each attack is individually targetted and each can suffer from a mark. Multi-attacks aren't that rare; changing this rule represents a significant nerf.
Indeed. So if you have an attack that requires two targets, and it makes one the defender to avoid the mark, it has made that choice. If you say that it can't attack the second target without triggering the mark, by the rules it can't even use the ability at all without triggering the mark. (See that other discussion for a lot more detail)
More than 139, since my first search term of three possibles turned up 139 hits. I wouldn't call that rare, but I'll grant it's uncommon

See that other thread for examples, they're pretty good ones.
I'm not sure
exactly what you're referring to here. Multi-attacks are common. Multi-attacks which
require two distinct targets are rare. For instance, people in this thread referred to dragons; well, a quick check shows that dragons' "Double Attack" can make multiple attacks vs. one target. Blue dragons even have a "Draconic Fury" which gives them three attacks - again, which can be focused. White Dragons have "Dragon's Fury", which has a clause which is again common on many creatures - if the dragon hits the same target with both attacks he's extra nasty.
Clearly, most multiattacks can target a single target, and clearly, that's not an accident - it's not uncommon for monster powers to explicitly depend on that ability, too.
I'd also like to stress that solo's aren't the only multiattackers under the sun, there are many others that can do so too. The solo's problems with respect to effects aren't specifically related to marks. If the motivation behind a rules change is to be that solo's are getting pwned, then marks would not be the first thing I'd jump to fixing.
Now, the _real_ reason to have melee/ranged apply mark each attack is because you can do different things to each attack and interrupt them partway through and disrupt movement and such. I've never seen it happen, but it is possible to declare a defender as a target and then never manage to get the attack off.
These are the exceptionally problematic situations.
I will note that my main objection is for multitarget attacks, not multiattack. Ie, attacks which _require_ more than one target but are melee/ranged.
I think the base rules are sufficiently clear on the matter. Changing the rules concerning multi attacks changes balance and weakens marks for no particularly good reason; some of these multiattacks are quite strong, and it may matter a lot whether the multiattacker can use one of three attacks on the buffed defender and the rest on the weak, less armored dude that perhaps even grants combat advantage next to him. It also opens up a can of worms in some corner cases. The number of multiattacks that actually
need to attack multiple distinct targets are
small and I'll wager that even most of these have a reasonable (if less attractive) fallback option which can target just one creature.
Basically, I ask "why would you do this
in general"? If you find a few specific cases of poorly designed monsters where you think that the attacks were really intended to be played as a kind of mass-damage area thing (and it actually matters much), then
of course the DM should choose to fix those design issues on a case-by-case basis. But what you're suggesting alters far, far more than just those cases, it alters
every multiattacker, even those that clearly
were intended to (be able to) target a single target usually. When the paladin stands guard next to his fallen friend, the
intent is that his mark distracts the critter from attacking his allies.