So the problem with PC-built monsters are manyfold.

First, they have ridiculous alpha-strike capabilities for their toughness. All of those daily powers can do some serious hurt.

Second, they have abysmal HP, and when 5 players concentrate fire, you need multiple heals/round to keep yourself from going down.

Third, they are designed to work in a group even more than monsters are, and feed off each other.

Forth, they are lots of bother: you really don't care why their defence is 18 vs Reflex attacks. The PC rules are full of 'busywork' so a player can tweak this, that or the other thing and enjoy building their character: NPC rules are fast and loose, and generate similar output while skipping the details.

Basically, you can generate a similar effect by using the NPC/template class creation rules, instead of using the PC rules.

Solos, with their high HP and reasonable defences, give a pacing mechanism for the fight to last 'long enough' but not too long, while (hopefully) generating enough threat to make the result of the fight in question.