The big amps aren't for everyone

Not everyone needs a 100W monster of an amp.

Size shouldn't equate to functionality

Guitar amplifier manufacturers seem to follow the same logic that car manufacturers do when it comes to functionality. I'll make the comparison based on an amp's watts relating to a car's engine capacity.

Now think if you want all the latest mod-cons, then you have to purchase a car with the largest engine size. It's the same with guitar amps. You want direct recording, speaker emulation, valve switching, multiple channels, etc, then you'll quickly be looking at a 100W amplifier.

Let's say you look at the 50W amp in the same range, then you'll notice that some of the knobs or switches will be missing. Look at the 25W and it will appear bare. The manufacturer will not be offering as many functions on the lower wattage amplifiers.

So if you want functionality, you end up having to buy above your needs.

Who actually needs a 100W amplifier?

I don't know the answer to this. I can see why 15W isn't enough for clean headroom where there's no PA system or other sound reinforcement. I can also see why you may need 30W, depending on the gain structure and channel settings, again only if you want a nice clean sound and great overdriven tone at the same volume.

But I can't see why you need more than 30W.

Is the amp for you or the audience?

If the amp is for you, then it only needs to be as loud as the sound on stage. 15W overdriven will be enough for playing with even the loudest of drummers. If you want a crystal clear tone, then you may need a bit more.

If the amp is for the audience, then mic it up and put it through the band PA where it can be as loud as legally permitted. All you'd need is a Shure SM57, mic cable and stand. And with that, you can get away with a smaller, more portable amplifier

My take on the matter

Give me a small amp with great tone any day and I'll mic it up. Monster amps look good and they have their place on stage in stadium gigs, but I prefer small.

Have a look at some of the smaller amps on this site, you may be surprised.