(5.5E) Is the 9th level Rogue Trickster ability Magical Ambush severely limited due to ONE word used in description?

Edition - 5.5. Magical Ambush is a 9th level Arcane Trickster ability. It reads "If you have the Invisible condition when you cast a spell on a creature, it has Disadvantage on any saving throw it makes against the spell on the same turn.".  I am assuming that invisible does NOT equal hidden. A Rogue can use a bonus action to get hidden often. Invisibility obviously requires a ring or spell, etc. And if you use it that way, it just seems to be a waste of resources.

I am going to make the case to my (fairly reasonable) GM that this 9th level ability is severely limited and could we consider allowing it to be "hidden" instead of "Invisible".

How would you rule? What am I missing?

The hide rules are some of the more hotly contested ones in the edition. I expect lots of people will chime in on this one. I’ll try to cover it as neutrally as I can.

If you are obscured, or behind cover and make a dc 15 stealth check, you gain the invisible condition; the hide action makes you invisible. So far, it’s pretty easy to say the power would work well. BA hide, hit your DC, action, cast your spell and the target has disadvantage. 

There is, however, a lot of debate about it. Basically, if you go around a corner and hide, as a result you have the invisible condition. What happens when you step out from behind the corner?

Theres a bit that says using the search action is the only way to find an invisible creature, because that’s what the rule says. Since you are invisible, you could walk right up in front of the guy, do a little dance, moon him, whatever, and he can’t see you unless he searches. 
Others say, that when you break cover, the target would certainly see you, because, well, you might have the invisible condition, but you’re not actually invisible, per se. You just hid really well, and once you start walking around, you’re not hiding anymore. 

So, it will be up to your DM to decide how to run it at your table.