Main pro is it let's you produce more kgms on your existing land, which will almost always be a long way more profitable than rearing heifers instead (bar your doing something like ZG outblocks etc). It also simplifies your system, you only have cows. Final advantage is in the current market its cheaper to buy them than rear them yourself.
Biggest single disadvantage is the disease risk. Only way you can reduce this is have a very good history of the stock your looking at buying in. Any other risks aside from that are very minimal, one risk is the market swings against you, and you have to pay big money for your heifers, however these has been largely an excess production of heifers the last few years, so unless you leave it very late this hasn't been a problem. Another risk is you buy in poor quality heifers, however let's assume that you have the cop on to spot a good heifer from a bad heifer and you only buy in something that will be close or better on what you can expect to breed yourself in terms of quality.