DaveO

I think that muscle power is a bit misunderstood sometimes. It has distinct advantages and disadvantages but each of those vantages are dependant on situation, use, or viewpoint.
Strong - not necessarily big - muscles are a huge advantage in a physical confrontation. They provide movement and control, this is true, but they also provide mass and protection. The upper chest for instance is protected not only by a strong bone framework; it's also covered by layers of dense, impact-absorbing muscle.
Strong muscles also aid in both speed and quickness and can provide a powerful backup to one's skill in an encounter.

All that said; physical strength has its limitations. First; the armour question: while muscle protects much of the body, it doesn't protect all. The body is covered with AVA's (anatomically vulnerable areas) that are either not covered by muscle (as in the testicles) or are themselves part of the musculature (e.g. the sternocleidomastoid or quadriceps). Next, structure: Muscles work by moving the body's framework - the skeleton. One of the reasons aikido is good against hard-stylists is that when done properly it acts against the structure; not the musculature. Big buff guys are surprisingly easy to topple, twist, bounce and otherwise discombobulate when they rely on their muscles because they forget - or don't know - this fact. Another problem is that muscles only work in one direction. If one is using muscle; he's concentrating his energy into one particular muscle group. In order to change his motion; he's going to have to release that group, reorient and fire another. For instance; witness a power puncher in a bar - he throws a great big shot; relying on strength to get that fist swinging. If the target suddenly moves; he has to check that swing, readjust, get the appropriate muscles set up again and fire another punch.

The BIG problem with muscle though is that it's largely used as a patch for improper technique. People who don't know what they're doing - or don't know enough - forget about the Big Three of defensive attack: structure, range and positioning - and try to launch attacks from wrong locations and placements. Since the attack won't work properly in this case; they use muscular strength to try to close the gap between ineffective and effective. This is where you see overreaching punches, off-balance strikes and grunting and straining through a shihonage.

Finally, muscles in aikido practice:
Bad, bad, bad. Oh; not because there's anything wrong with muscles; but because of the above point: if you're using muscles; you're doing it wrong. Remember; like any martial art aikido is a training system. That means it has a series of techniques and drills that teach specific principles of attack and defense. (Yes, aikido has attack. In fact; the majority of aikido is attack. It doesn't have true strikes, but that's not the same thing.) See; in the dojo if you're using muscle to force a technique what you're doing is trying to cheat your way through rather than do the technique properly; learning the principle it's supposed to teach.
So: Muscle - good. Has limitations.
Muscle in aikido - bad. Enforces limitations.

One more thing:
Leon said:
Wow - Leon; this is a bit tricky since I'm not entirely sure what you intended to say. But it looks to me like you said you're being corrected for using muscle when you know you're not? If that's the case; boy do I know where you're coming from. This is a place where big guys really need to go slow and look closely; because we're really bad at determining when we're using muscle. We're so used to being strong we use strength by default. There is a big, big difference between using 'no muscle' and using 'less muscle'. I suspect you're using less, not none. The second quoted sentence would seem to back that up.
Leon; I strongly recommend that if you want to get away from using muscle - and trust me, you do - start practicing really, really slowly. Walk through techniques at a snail's pace; looking at every movement and seeing where you are applying muscular force to your uke. Once you do that and can point out where you're muscling uke; you'll notice your aikido taking a big jump forward.

To answer your last question; no, being powerful in the muscular sense is in no way wrong - it's very good. But by learning to use structure and not muscle, what you will be doing is learning how to get the most out of your muscles. Even when you're using every ounce of strength - for instance in a dead-lift or push - you are in fact using only a fraction of your muscle's maximum potential. Learning to use structure unlocks the rest.

Cheers!

Dave