Is there any reason not to do laundry with bar soap?

Hand soap and laundry soap are two different things. What you want to do is to make the water "thinner" (destroying it's natural surface tension) so it gets the dirt out. You can get the same effect with pretty much any soap or dishwashing liquid. However most powder laundry detergents also have some whitener in it that make your clothes "appear" white and clean, and this is something you don't have in hand soap.

What I usually do is get some paper or magazine and an empty & dry 0.5L fruit juice bottle (or anything with a big bottleneck and a screw bottle cap) and then funnel some powder laundry detergent into the bottle using the rolled magazine or paper. This way it's sealed, quite compact and won't make a mess in my luggage when it gets thrown around on luggage belts. I do this mostly because a lot of hostels charge you $2 dollars for 1 small bag of powder (enough for 1 wash load), while you can get a small box of powder (~20 wash loads) for $4 in the supermarket.

I am a permanent traveller, and I wash my laundry with hotel bar soap or shampoo. Some is hanging over my hotel balcony to dry now, in fact. I've never ruined anything with it, and I've been hand washing silks and knit wools regularly. In my opinion it is more important to be careful in not using too hot of water, wringing or hanging things that shouldn't be hung to dry. Though with bar soap in general you do want to be careful not to use too much (i.e. don't rub the bar directly on a thick knit) or it can become difficult to remove.

I mostly do this with my socks, underwear, tops and other easy to wash items. Then I plan to use a washing machine every few weeks for items I wear many times without a wash like denim and jackets. For this I carry these:

GrabGreen Laundry Detergent

I keep them in their plastic bag in my hardside suitcase, and have never had any troubles with breakage or mess. If I am going on a short side trip, I bring one "just in case" in a small plastic bag. When it is laundry day I toss one or two into my laundry bag to take to the washer.

I agree with Mouviciel about the validity of Aleppo soap as an all-purpose product, but I find it a bit too expensive (at least here in Italy it is normally marketed as something posh), so I would recommend Marseille soap. If you buy it pure, i.e. with no perfumes or other things added, you can really use it for everything, and it is less expensive than the other option.

Or else, if you happen to be travelling through Southern India, you can always find small bars of laundry soap, often marketed under familiar brandnames, which are extremely reliable to do the job, easy to carry around because of their size, and absolutely cheap (5 rupees or 10-15 cents).

The washing powder has some advantages:

• You can't use soap in washing machines (it generates too much froth)
• It's much less labour intensive to use washing powder even manually (tested with 2 kids last month, when both of them had all their clothes completely dirty every day)

but it obviously is a pain to carry. If you are just travelling on your own and you are not planning to use washing machines on campsites/hostels then probably soap will be fine.

Although it is not as popular in the US, special bar "soap" specifically made for doing laundry does exist, and is often carried at common places like grocery stores--it only seems hard to find because it's so small and the demand is low so it may be on a top shelf where you can easily overlook it. Look for brands like Fels-Naptha, Octagon, Lirio, or Zote, or search for a resaler online: http://www.google.com/products?q=bar+laundry+soap

I would expect laundry bar-soap to work significantly better than hand-soap should bringing liquid or powder laundry detergent be non-desirable. You really want a detergent more than a "soap" per-se, glycerin isn't going to get clothes clean, so depending on the main ingredients in the soap it may not be effective for clothes.

When I travel light, I carry Aleppo soap and I use it for everything: skin, hair, shaving and laundry.

I never found something better for that many usages.

There is really no real reason why you can't use bar soap for laundry (at least for hand laundry.) It wouldn't be practical at the laundrette/laundromat because you'd need it either flaked or in liquid form to work with the machines (a little grater might help with this; grate into powder and either "melt" it in some hot water to make a liquid detergent or leave it in powder form and put directly into the allotted space in the machine.

Why not? They do a pretty good job too. I ran out of laundry detergent while away for a 2 month travel and unfortunately nearby shop ran out of laundry detergent (it was in an isolated area).

I substituted by using Lux bath soaps (they are all that's available). Leave my clothes smelling good and got rid of the stains too. I rub the soap on the wet clothes, scrub and scrunch them a little, leave them soaked in the sink for a while and rinse. My clothes smell great once they dry :D

You can buy sheets of laundry detergent. The ones in the supermarket usually have fabric softener in them too, which is ok if you're going to use a washing machine. For the sink you can use something like Travelon sheets. They appear to be scaled for sink-sized loads (too much soap makes rinising a big challenge) and packaged for traveling. Not the cheapest option, but handy.

The Amazon page is rich in links to other handy things too: sink stoppers, in-room laundry lines, inflatable hangers, ... who knew?