I think a lot of what can be achieved using VHOs, can be done with T5s. The Super Actinic is available in both a T5 HO and a T12 VHO.
When it comes to actinics, which is what people most often use VHOs for, you're not necessarily after a very high intensity-- you're just after a little extra coloration. While it's true that T5s are more intense per watt (especially when combined with SLRs), they also cost more money-- the reflectors cost more and often the ballasts cost more. Also, as Bill mentioned above, VHOs have built-in reflectors which further enhance their 'small' profile.
That being said, the lower price of VHOs, while at the expense of being less efficient (less intense), is the primary reason most people go that route. GreenMako (when he had his tank) used VHOs alongside the primary lighting from MHs. If he wanted to, he could have easily substituted T5s in place of the VHOs, but since the function these bulbs play is minimal, why bother?