Is there any desvantage of desloratadine in allergic rhinitis with co-existance asthma over other antihistamine

There's no real disadvantage to desloratadine (other than the fact that in the US it's still a prescription). It's non-drying (which is good for Asthma), non-sedating, and lasts for 24 hours a dose so as long as it works for your allergy symptoms it should be fine, the main thing to think about is whether it controls your allergy symptoms (be cautious of older antihistamines that are drying, they can aggravate some people's asthma).