Pros:
1) Keep the interest payments high over the term of the loan
2) Handy for cashflow purposes
Cons:
1) No capital repayment at all - you will always owe the original mortgage amount
2) Depending on the future sale of the house to make capital growth profit.
Tax deductible in full at top rate .. would be one reason;
In essence, if the only way a 2nd property works for folks is on an int only basis, then you are relying on continuing capital growth ..which will stop one of these days.