for plant health. and I suppose because I'm not one to split apart a plant unless it's absolutely necessary. it may take quite some time for a new basal keiki to grow sufficient roots, and if it is getting much of its nutrition from the mother plant, its growth may decline greatly if separated. now, that's a two-way street, sometimes it sucks so much energy from the mother plant that separation is indicated. but when i say i like to keep things together, i mean more of a healthy plant that is bursting with so much vigor that it throws a basal keiki, in which case leaving them attached gives a real spectacular show after the basal keiki grows up. and if the parent growth isn't healthy, then maybe the plant is putting all its energy towards the basal keiki, and i'd rather that every bit of energy that the plant has left is directed towards the new growth (which will form roots when they are needed, usually).

there is also one thing that i noticed that makes me hypothesize that perhaps new growths may produce some hormone that the plant needs. i have absolutely NO proof of this, but this is just an experience that i will recap--my mother had a plant of phal. taipei gold 'stm' that was a strange sight. it was about 8 growths in total of various size, growing from every which direction. i had no reason to think that multiple plants were potted together, and it seemed that this particular clone had a bunch of basal keikis. she asked me to divide the plant, which i did, into two roughly equal pieces. both had good roots. one of the pieces went on to grow normally, while the other one just stopped growing. no rot, no bugs, but the thing just stopped growing and eventually started dropping its leaves one by one and died. that's what made me suspect that the piece that lived, for some strange reason, was producing some growth hormone or substance that the rest of the plant was relying on. again, i have no proof of this, and have no reason to suspect that any other plant would do this, but that's another reason i like keeping things together.

another good reason: dividing decreases the amount of room you have for more orchids. a big pot takes up less room than a couple smaller ones.