There are several reasons a slant is best for culture storage (and none of them are real scientific!):
1) Surface area - by making the tube at a slant, you increase the surface area as opposed to just a flat surface;
2) Size - a test tube is much easier to store than a beaker - you can fit nearly 20 test tubes in the same area as one beaker;
3) Moisture - plates, when stored, tend to dehydrate very quickly. A capped tube will retain moisture much longer;
4) Ease - while it is a pain to make slants, once the microorganism is transfered, you put it in a frige (2-8 Celcius) and leave it for up to 6 months. If you used a plate you would have to transfer every week; a beaker would mean transfering every month (and they spill so easy!)
One thing to remember about slants and storing microorganisms -- the more often you transfer them, the less like the 'original' they become. If you are working with a specific microorganism because of a unique characteristic try the following:
1) Grow a very heavy inoculum of the orginal on a plate with approriate medium.
2) Make several (5-10) slants from that plate.
3) If you plan to work with your microorganism, transfer a fresh culture from one of the slants to a couple plates.
4) Use those plates for about a week and then get a new inoculum from the original slants.
In general, do not transfer more than 3 times from the original culture!
I am a Microbiologist.