Haven, I sympathise and empathise with you. DD is in a similar situation. She first developed a wheeze at about 15 months, and has been in hospital six or seven times now. She doesn't seem to have any triggers other than a cold.
I don't know if these children are more prone to coughs and colds, but anecdotally, my DD seems to be (her 4 siblings are rarely bothered by colds, and she's the only one that doesn't mix with a lot of other children).
The major step forward in the last year was when DD was given a steroid inhaler (Becotide - the brown one). She has had a cold this last week with no wheezing, which is quite amazing for her.
I think you just have to accept their condition as a fact of life for at least the next few years and hope that they grow out of it. Asthma seems all about how well you manage it, and if you are strict with the medication, then the quality of life should be very good.
Pneumonia is very scary - DD had this last year (she was 22 months at the time) and it brought on the asthma attack so suddenly that she was unconcious by the time I got her to A&E.
I used to think that asthma was no big deal, but having a child with it changes everything. It is a very life-threatening condition. DD's asthma attacks are fairly predictable as they always accompany a cold, but it has still affected us quite dramatically: I was planning ot return to work this year as a part-time teacher, but realised that I couldn't - it wouldn't be fair to the school, and I'd hate to be in a position to delay any trips to A&E.