Justify me a PSP?

I have an opportunity to get a PSP for $150 that's in semi-used condition.  I have a DS and don't use it all that much, but I prefer that system better.  I'm not too big on most of the titles coming up, but if someone can name me THE #1 REASON to get a PSP (I already have an iPod and laptop, so no need for connectivity, all I want is teh gaemz) as Halo was for the Xbox and all the decent RPGs are for the PS2, I'll probably end up getting it. 

The catch is that I'm about to spend $80 on Osu! Tatakae! Ouendan and Cooking Mama, so it's going to be one or the other.  I know that $150 for a PSP is a very good deal, but I don't see myself using it as much as my DS, which is only on long car or shopping trips (the girlfriend takes some time to try on clothes and shoes) or to feed and take care of our Nintendogs.  I like the DS for what it can do with the interface, but the PSP is the powerhouse and has MG Acid, which I wasn't uber-gung ho to play, but would if I could.

I have no plans to watch UMDs, and while I'd like to put old ROMs on the thing, it isn't an absolute necessity, so firmware isn't a concern. 

So?  Should I go for it?  What games are absolute must plays that are only on the PSP?

Honestly, it's a cool accesory. Thats it really. For right now at least.

Lumines is great, still playing that. Same with metal gear acid. My friend has socom for it and says its fun, the megaman x remake is great(only $30), and that monster hunter game is something im keeping track of, as is breath of fire 3.
Also, if youre a metal gear fan like me
http://media.psp.ign.com/media/791/791367/vids_1.html not a game, but i think it's awesome. Love the artwork in it.

Oh, and if all else fails, homebrew. And porn. Lots and lots of porn.