I level'd a WF Wiz18/Rog2 before. (and also pure wiz, monk splash wiz) and I think rogue splash is easier to level.
Some quests require or much better with a rogue. e.g. Tear of Dhakaan. With rogue level, I don't need to wait for a rogue to join the party. I usually play with very good players in the party, usually play same or under level quests (i.e. won't do very challenging quest), and we split up to complete the quest to gain the highest xp per min. With rogue skills, the group could complete quests in roughly the same time with extra distrap bonus xp.
Other than Tear of Dhakaan, there are many other quests that rogue skills is highly desirable, e.g. the rat maze in necropolis part 3, the one in necroplis part 2 with high disable trap bonus, von5 etc.
It's true that you'll get new spell later. For me, I actually took wiz1-7 first before I take the rogue level. As a TR, I concern spending less time in leveling more than max out skill points. Not taking rog at the first level means u can max out one less skill. For me, it is not a big deal as a wiz has more skills than i need anyway. Assume you will take rog 1 and then wiz 1-7, you'll get Fw at level 8. For a non-TR, I don't think it's hard to level up from 7 to 8, and after you got fw, you will use it intensively until you arrive Vale. I don't think getting new spells one or two level behind a pure wiz is a big deal. And Banshee isn't a game breaker.
For new player, i would even suggest to consider taking the 2nd rogue/monk level earlier. (i definitely won't do it myself, however) Evasion is useful to reduce damage and survive better. After you get FW and be able to use Reconstruct scroll reliably, there is no big deal to get new spells later.
I don't like rogue splash personally as I think disabling trap is boring and stupid. but from my experience, they could level up faster.