After playing a lot of 4E, I'm quite ready to go back to spells in stat blocks. There are some major advantages with the system, of which the main one is that it gives the creature versatility.
One of the problems I had with 4E was that monsters were very limited in what they could do. Typically they had one main attack and one special attack. The trouble came when the players did something to negate the special attack - at that point, they lost most of their effectiveness. Having a list of spells allows monsters more versatility depending on the situation. A monster that can cast thunderwave, web and fireball is more interesting than one that only thunderwaves.
Of course, I also detest having to memorize hundreds of spells to use them with monsters... but, with any luck, most monsters will stick to a relatively small list of spells which we'll memorize easily because they're used so much.
The things I really wanted eliminated from monster stat blocks - and doesn't appear in 4E or 5E - were feats. Feats are tremendously more obscure than spells, and are a major pain in running 3E and PF.
One minor benefit of having monsters use spells is that, as the DM, I get to read spell descriptions rather than just taking my players' word for what spells do...
Cheers!