There are two big downsides to high level limited events and they are linked.
First up is the coverage, I love limited but I willingly admit that it is less interesting to follow along with. The flow of games in more random as most cards are singletons in their respective deck. Looking at winning decklists is also dull. Watching drafts can be cool but that is about it.
The second major stumbling block is connected. The Pro Tour has one over-arching purpose to help WOTC sell Magic cards. Constructed is amazing at this, Tamogoyf rocking every format known to man increased the demand for Future Sight boxes. People watching at home check how many Figures of Destiny to build the latest deck. Limited does do this. No one sees a winning limited deck and says "man I should buy some packs to draft with."
Combing both formats in every PT gets around these downsides. Even in the PT with a t8 draft each year there will be undefeated and t8 constructed decklists. The constructed will make coverage more interesting and help Wizards move more cards. I also think combing formats will make the PT more interesting and challenging.
Many Magic players are regarded as limited or constructed specialists. The best of these cross over. Two great examples are Wafo-Tapa and Kenji. Both were regarded as some of the best constructed players in the world. Their limited game was not bad but it was not where they were making t8s. They both however changed this. Kenji improved so much he was voted into the 2007 Invitational as the Limited Master.
Combining formats forces players to adapt or fare poorly at PTs. This I think is good, PTs are the highest level of Magic. You can still do well at GPs with just constructed or limited skills but to be the best or the best you'll need both.





